So I don't know your situation but I will say as someone who was 20 all of 5 years ago and who now works with 19-21yr olds, I don't think your feelings are necessarily unique (as unfortunate as that might sound). This is often a time of transition for those of us who are privileged enough to have compulsory secondary education, either going into college, uni or work, which can be very daunting and nerve wracking.
Youth (people aged up to 30yrs) face a number of distinct struggles that are foisted upon them when they have hormonal imbalances, limited experience and lower brain development. Feeling alienated from others and distressed is not a unique response.
This is part of the reason why Communist Parties have Young Communist Leagues, which are (ideally) responsive to needs of their younger membership. I would consider joining one of those if you can/haven't already. You might find that joining a YCL or a club with a similar purpose might help you "find your tribe". This should not be seen as the limit of your social escapades and making and maintaining friendships with non-communist friends is important. Some of my oldest friends are anti-communist but the time I spend with them rejuvenates me because we do things and talk about things enjoy and they genuinely care for me.
Similarly important to your social wellbeing (vis-a-vis your mental health) is ensuring you are eating, exercising and sleeping properly. Considering that dialectical materialism argues that a material base determines an ideological superstructure, it is indeed communist to maintain your physical well being (as evidenced by the emphasis placed on this in the Soviet Union).
Make sure that as you are going throughout your day that you are taking note of how certain interactions and events make you feel and give yourself some time and an outlet in an attempt to release the stress that builds up. This can be journaling, poetry, meditation, talking to someone you trust, going to the gym, playing a sport, going for a walk, listening to calming music, volunteering, watching a funny video (fun with friends and family), playing a board/video game (fun with friends and family). A combination of any or all of these is recommended so long as they wok for you.
Some distressing events can still be affecting you even if they happened a long time ago, especially if you are idle (one of the curses of unemployment). I went through something distressing this summer and it still bothers me although this has lessened since I started to fill my schedule up with school, work and Party work.
Krupskaya, writing in 1932, proposed that a YCL must wage a struggle for knowledge, necessitating serious knowledge for socialism, the ability to apply this knowledge, an understanding of Marxism-Leninism and a capacity to work with the methods and in the spirit of Marxism-Leninism. This is a never ending process, as a comrade much more experienced than myself told me: one does not join a Communist Party a communist but rather to become a communist, a process that never truly ends. Thus your task at this moment, as I see it, is to learn more about the world while participating, as you can, in proletarian and youth institutions.
Be kind to yourself; even if it doesn't seem that way, you are needed by me, by the others here, by your friends and family, and ultimately by the international working class.
