When I was younger I also was told that I had a bit of a "debbie downer" presence.
Something that helped me was the concept of "yes and", when someone says something, find anything in, or tangential to, their statement that you can either :
- Agree with
- Comment on
- Tangent from
Then the "and" part, add something of your own to the conversation.
Force yourself to balance your natural negativity with forced positivity and over time you may find that you no longer have to force it, and that you enjoy finding hidden positives in things, like a game, it turns conversation into a puzzle.
In general, people like being agreed with. So sometimes before disagreeing you can even say, "I agree with X part" and then instead of saying you blatantly disagree, change your tone to something like, "I would probably modify Y and see if I like it." It's all about controlling your tone and playing to human tendencies.
Once you get to know someone better, you can be a bit looser, some people like autistic truths. Know your audience, sometimes it's as easy as asking, "Oh, am I being too negative, I try not to do that."
Good luck!