I have had many years feeling strange, rejected, and alien in social situations in and out of school. I wish I could tell you how to never feel that way. If you ever find out, please let me know. I am very fortunate. A year and a half ago I got involved in an informal in person support group. It took a ton of courage for me to go. i didn't really say much for awhile.
However, I met my best friend there. In my whole life, I have never really had a best friend and rarely really had anyone i could call a friend. He also battles mental illness.
normally social interactions feel to me like I am an alien on the planet Earth. I know a bit of the language, just enough to be thoroughly confused.
But talking to my best friend and also the lady who ran the group is like conversing like someone from my home planet. Honestly, I didn't think it was possible after so many years of isolation and loneliness to truly connect with anyone else. But when I went in the hospital in December they were both right there. They visited me, went to the store and brought a bunch of clothes for me to wear, and lots of reading material.
So I guess what I am trying to say is although you may think you are always alone and rejected, there are also a lot of other people feeling that right now. I would guess a lot of people on this board for example. Even though you are busy with school and scared of groups, I would encourage you to seek out people who share something in common with you. Whether it be mental health, comic books or soccer, there is always someone who is passionate about something you are. Then you can focus on your shared interest and work into talking to people. For instance, I have gone to a lot of conventions for fans of tv shows and movies. For a long time, i never spoke to anyone. But over time, I realized that I had much in common. Frankly it took years of work to get the confidence to even starting smiling at other fans. But that common interest eventually provided me with social interactions and a testing ground to practice how to speak to people. Although i didn't make lasting friends, I learned to enjoy human company and find out the most everyone, if they are honest, feels scared, lonely, awkward, and isolated.

I hope this helps.

DW