Is it me or something else..?

Okay well I'm 15 and I never feel good enough, for anything. I always feel like people are trash talking me or planning ways to torture me or something. When I'm with people, I always wonder if there's some other motive than just to be with me. Whenever I make a mistake, no matter how small, I always feel like I have no reason to make a little error and I constantly feel like I need to punish myself for it, which I do. I used to have issues with bulimia and I still have issues with cutting myself. I don't feel like I have anyone to talk to about anything, so I just keep everything inside and eventually I break down. All the time I go from idolizing people to hating them, and I'm not really sure why. When I'm around other people I act all happy and stuff but I really just want to scream and flip ****. People always tell me that I'm always so happy in a good mood, and I wanna rip there heads off becuase I feel like there's something wrong but I'm not worth listening to.The only time I can really calm myself down is when I think of one time when I cut myself and the blood flowed out steadily instead of just a little trickle, which I'm sure isn't "normal." I feel like if I would tell someone any of this, they'd say I'm overreacting and I need to relax, but I don't think I am. Do I need to step back and reevaluate the situation and stop terrorizing myself or is something else wrong?