Here are some thoughts based on past reflections I’ve had (so they might not be phrased the best) in no particular order:

First, you mentioned sometimes not understanding what is being said. I minored in philosophy during college, and the first and most important thing I was taught during Intro was the principle of charity during discussion, composed of two parts. The first, assume the other person has something worth saying, isn’t super relevant here. But the second part is that questions of meaning come before questions of truth. If you can repeat back what the other person tells you in your own words and have them agree, then you can start forming a rebuttal.

Speaking of forming things, I believe that the thoughts you express are not your thoughts, but a linguistic representation of your thoughts. When I envision a thought forming, it’s more like a glowing ball of thought and words can be put together to form a kind of shell around the thought. The better the wording, the closer the shell gets to matching the shape of the thought itself. Improving your English toolset will allow you to better shape your words to match your thoughts more quickly and easily. The primary way to do this is to expand your vocabulary/lexicon/internal thesaurus and ability to connect words together, and the primary method to accomplish that is to expose yourself to new words in new orders, and you do that by reading new stuff.

All that is just the prep work. In an actual conversation you don’t have the luxury of reading a bunch right now. My advice there is that, if the other people are cool, be upfront that you are still formulating your thoughts into words and ask for help in finding the right ones if your struggling. Doing so will leave less space in the minds of others to assume anything else, likely leading to not just more patience from them but possibly help for you as well.

I think the process of forming thoughts into words is one of the most fascinating things there is. The times when I have observed myself pulling an original thought out of the ether of my mind or better yet being able to help someone else in pulling their thoughts out of their head and into the world are some of the most beautiful moments I have experienced. I wish you well in working on your ability to find those moments!

Practice explaining things to others.

Pause a lot during your explanation and, if you can, get them to parrot back what you're explaining so you can see how they are understanding it.

Ask them questions as you explain it.

Draw it out or use some kind of graphical way of showing what you're talking about.

Try to explain it in a way that accounts for their motivations.

I'm with you.

It seems like more times than not, I feel like I've fully explained the situation, and the person I'm talking to responds as if I said the complete opposite or something completely unrelated.

I'm willing to accept that I'm doing something wrong, but when I put extra effort in to make sure that everything is explained accurately, it STILL gets misinterpreted. I'm starting to think that people are just really stupid and/or terrible listeners.

I would suggest a few things. First, be genuinely open minded to the discussion/topic (as in your opinion can change with a good enough argument.

Second, I would suggest using "feeling" language as much as possible. Examples are: "I feel like that will cause an issue with...", "my understanding is...", "my thoughts are...", "from what I've seen/read the setup is ...", etc. From my perspective, feeling language lowers the "absoluteness" of statements and makes conversions easier by changing them from arguing absolute truths, to discussing lived experiences.

Third would be to reframe the discussion/argument from "you vs them" to the two of you vs a problem/issue and you both are working together to find the answer. This somewhat goes with being open minded.

Fourth, I don't think works too well in online discussions, but in person I find starting from a base situation and asking the other person their thoughts on it, and adding your own, works well. So, start with a base situation and then adding a little complexity each time you both agree on the outcome and reasoning. Once you've found your divergence point you can start having your discussion about the pros/cons of different approaches/solutions to that specific situation.

Use the Socratic method; phrase your statements as questions.

It helps soften your dialogue quite a bit and makes the conversation a quest for discovery.

It's not necessarily your problem. Especially on the internet.

Don't let the opinions of strangers who are often willfully and ignorantly interpreting you dictate what you mean.

If you have certain discussions you want to be a part of, well read up on that topic, get informed. And choose sources, which, as you say, are interested in understanding things, rather than proving a point or 'winning'.

They exist. But boy oh boy is most of the info out there some weird petty dummy who just wants to beat up on anyone else who has a different opinion. And most often this is the type of person you will encounter in discussions, both IRL and on the internet, and the best response to those people is to ignore them and focus your energy on folks who want to engage you in the way you want to be engaged.

I know I do many things badly because im just winging it off without enough contemplation. Honestly what works best is when you have two people actually conversing and asking things they are confused by and responding to questions in a straight forward manner.

I will warn you that there do exist people who will deliberately misunderstand you because there is someone else that they are arguing with and you just happen to be present, and no clarity of expression will get them to understand you. How you choose to deal with these people is up to you.

The best thing I did to improve my ability to speak persuasively is learn to write quickly. I studied literature, and it was common to take exams where you have to write a short essay in a quick timeframe in response to an excerpt from something you studied during your coursework. I spent a lot of time writing a few paragraphs making a point as quickly as possible, and then read back what I wrote and try to figure out how I could be more persuasive. It bled over into my speech and I still speak in paragraphs.

I don't know how to grade myself on clarity of communication, but I'll share some things I think have helped.

Avoid long sentences. Many people don't read well, and even those that do will get lost in a long block of text.

Read back what you wrote out loud.

Do not neglect the emotional front. Conversations and convincing are about emotions more than facts. Acknowledge their position. Make them feel heard.

Read more. Read a variety of things. Op-eds in newspapers. Books of various genres. Old stuff. New stuff. Feed your brain more than word junk food.

Ask questions. Sincere questions. Don't try to trick them. People are usually happy to talk about themselves and their beliefs. Use this to find common ground and shared experience.

Rhetorical Triangle

Make sure you're appealing to your audience correctly. Typically people lean too far into one (Ethos, Pathos, Logos)

Easiest example is not overloading what you're saying with the (logos) subject matter / details when your audience is not detail oriented.

Another is the classic "trust me bro" when the audience doesn't know or trust the speaker. Invert that you have cult leaders whose audience trusts them absolutely.

Third example is someone who overloads emotional appeals to the point that the actual message is lost, misunderstood, or disregarded entirely.

Curiosity helps way more than it should in helping discussions come to clarity. Sometimes you just need to ask really obvious or dumb questions to make sure you have all of the context. And if you want to make sure you're responding to their point, rephrase their position as simply as you can without making a strawman. You might need to go back and forth a couple of times until you can explain it back to them in a way that you understand.

Once you really grok, then it becomes much easier to express your thoughts and opinions on whatever. If you aren't knowledgeable about something and it is relevant or key to the discussion, and it can be summarized and grokked on the spot, then simply ask. If you get it wrong, they will tell you. At which point, yeah, do some active listening and try to learn.

Honestly, the only way to really get better at this is to just do it. The more you roll other peoples ideas around in your head and explain them only makes it easier to take your own ideas and explain them. Personally, if I know the discussion topic ahead of time, I generally prepare myself to answer a few different rebuttals, and make sure I've identified any weaknesses in my position, assuming it is somewhat adversarial or contentious. That little bit of prep helps with the pre-meeting jitters which can be a distraction in you expressing yourself.

The only thing that's really worked has been years of practice. It's hard to say for sure we started with all the same problems, but struggling with formulating examples is the same.

Doing it in writing on Lemmy has helped a bit. What makes a Lemmy comment go far isn't necessarily what helps IRL, but the written format alone makes certain things easier. There's much less time pressure, and if you don't like a part, you can and do go back and reword it (as in, make it clearer, not change the meaning). You can also go back through the length of a thread, and reread it better than you can remember it.

Or is it really just keeping your mouth shut if you aren’t knowledgeable about something.

I mean, yes, that's always a good idea. Even better, go into listening mode and ask questions. If at some point you reach something you can comment on, which is likely, you can switch back into that, and they're more likely to listen to you in turn. There seems to be other approaches that work, because there's some very successful people that never listen to anyone, but that's mine.

If you directly try to argue about things you aren't actually qualified to argue about, if that's really what you mean, people will notice, and you'll just come across as insecure and unhelpful.

How often do you read books for fun?

I often rehearse conversations in my head, not so that I'm following some sort of script, but so I can think about what the person I'm going to be talking to is likely to say or think about what I've said. I've found it helps me listen to my own thoughts more through someone else's ears, helps me think about ways to clarify what I've said, and helps me tailor my words to the specific person.

"Or is it really just keeping your mouth shut if you aren't knowledgeable about something."
i often abreviate by saying "i know some stuff about X topic.." just leave it at that.
if they really want to listen then they will request my expertise. im not going to beg to abused while im trying to do someone a favor.
but first, i just dont conversate deeply with shitty people.
if someone has a good heart, then maybe they might not be the best conversation partner. thats ok. as long as they deliver on all the essential value, that i need from them.
complaining to the city clerk about various issues with the city. i think thats challenging and requires alot of carefully scripted+rehearsed fomation&responses.
i dont have a good answer. but maybe act kinda like a carefully controlled, unemotional, plan-adhering robot. (add back in some scripted emotion so that you dont sound like a robot.)