Happy Birthday, OP. Truly.

Wish I could say it gets better but I'm in a similar boat and life is just ups and downs for me. You might get more downs than ups, who knows..

My realization like this was at 23 (Just hitting 25) and compared to how bad I was as a teenager and losing everyone I cared about including family too, I understand.

We are tuned to 11 when the max is 10 when it comes to emotions. People aren't going to understand that. I trained myself to mentally bite my own tongue for my own sanity, and the sanity of the people in my life. You're lucky if you find someone who understands you and works with your outbursts or changing phases, but for the most part you have to accept you are this chaotic constantly changing being and try to win control. (You may or may not be that drastic, just an example from my own perspective)

I always had birthdays alone, too. Ever since I was 14, I think. Part of that was like you said, pushing away people near you for various self-created reasons or things you assumed about them.

Only way I learned to get better over the years and be happier, was to train myself. Understand why I say or do things and realize that maybe I should muzzle myself and avoid people until the storm passes. It's hard when it comes and goes often, but it's better to juggle emotions in yourself than be alone and suffer with that as well.

Have to train yourself to do the opposite with the sheer brute force of will. There really isn't much else that can help, aside from medicine, that's all you really can do. I live in my head with BPD, so the only way to make my life better was to make things in my head better.