Social anxiety and trust problems

I've had this problem for a long time, I've learned to hide it over the years because I thought I was just introverted and weird and that seemed off-putting to me so I hid it from others, but recently I noticed that the problem is deeper than I thought. I fear rejection by others, so much so that I can barely trust my closest friends, the smallest things cut deep and I just don't know why, I originally didn't want anyone to know about my problems but someone I trust recommended I talk about it so here I am, I'm kept up at night from time to time and just have this general feeling of being a burdon to people, like I'm just another random aquaintance to those I hold closest whom they could do without in a heartbeat, I feel replaceable at any moment, like if I just vanished no one would even notice or care, if I actually let someone get close enough to me that I think I can trust them the smallest things they do can sometimes shatter me to pieces eventhough they weren't even intentionally hurtful. I don't know why I'm so sensitive but I wish I could just let things go more, if someone is nice to me I used to get way too involved in the relationship only to be disappointed and broken time and time again because I apparently wasn't to them what they were to me, I just don't know what to do or how to keep myself from setting myself up for disappointment, my stupid brain always assumes the worst no matter what the situation, like if I'm left on read or something I will start to freak out and wonder what I did or said to make them suddenly dislike me, I consciously know how stupid that is and that they are probably just busy or have spotty internet but in the back of my head there is the constant thought of being on the edge of ruining every relationship I have. It's so bad that in public I basically have everything about myself from the way I stand, move, talk and everything trimmed to fit what I think will get most people to assume I'm normal and likable but deep down I'm just killed by the thought of being the lonely outcast I was when I was younger, but the only thing this has got me was a bunch of "friends" who couldn't give half a shit whether I exist or not, I'm a tolerated presence, thats all. I'm neither hated nor wanted, which is even worse to me that being a hated figure, at least people would have feelings about me then, but just accepting that I'm in this limbo space where no one gives even the slightest shit about me is draining me physically at this point, I'm the guy who is standing in a circle of people who are all talking and making plans for the weekend that specifically include everyone but me. I've tried dealing with these problems myself but the only thing that has got me is not seeing anyone anymore except a very few people whom my stupid brain still doesn't allow me to fully trust eventhough they are the best people in my life. That and several anxious breakdowns in my bedroom which have also lead me to finally reach out to more people, I hope this doesn't sound stupid but it's really draining me and tearing me apart so any advice or help is appreciated and welcomed with open arms, thank you for reading and hopefully replying.