well first of all, ur not at all the only one thats felt like that. ive felt like that before, exactly the way youre saying, and i know many other people have also.
make sure you get this 100%- YOU ARE NOT MEAN!! What ur doing is a vicious cycle, you dont feel secure so you want to act like you are, so you become friends with people just to say that you're friends with them- not because you actually feel the friendship- which is normal to do but ur probably a sensitive person who knows urself pretty well, so u feel like thats so mean to do, and then you end up being self loathing and because of that you arent able to be comftorable anywhere because you arent comftorable with yourself! So u want to pretend at least to be comftorable with other people, and then of course the cycle continues. I might be wrong about this, but from what you wrote, this is what it sounds like is happening, and thats why its been going on for so long. So you arent mean, its just that you're in a cycle of insecurity and self loathing, and thats whats causing these feelings. And also about the part that you said youre anti social- u arent! you might be more introverted though and rather a few much closer, deeper relationships then a ton of surface relationships which is perfectly fine!
Ok, now how to stop it. So I dont have any one quick fix answer (which believe me I know is extremely annoying), but some things that you can do, are
1- go in front of a mirror and tell ur self I'm special, I dont need anyone else to tell me that- do that every day, and after a while, ull actually start believing it. but say it out loud!
2- try doing one chessed a day for someone else, without telling them about it. it can be something really small, like picking up a peice of paper from the floor because you know someone else would have done it otherwise. It can be anything like that, as long as its something you wouldnt do otherwise. (that also stretches ur leadership abilities btw- because you're doing something instead of making someone else do it). If ud want to write it down in a notebook, thats great, and it works better if you dont tell anyone
3- make a list of people and things you're greatful for everyday, and go into specifics- youll see how many people really care about you from that.
4-thank yourself for something everyday
5- try finding someone- whether from school, an extracurricular activity, or even some extended family member like a grandmother, that you feel like you can develop a close relationship with, and grow from them, and work hard at it!
Good luck with everything and it might take a while, but if you perservere youll get there! I still struggle with these things sometimes, but these ideas really helped me break the cycle... good luck!