I hate everything about myself, and I don't know why?

Sometimes I scare myself with how low my self-esteem is. I don't even know where it comes from. I just don't think I'm good at anything. Like, I'm currently interning at a magazine, and the editor told me today that she read over what I've written so far and she really liked it. My instant thought was, "She thought it was crap. She's just trying to be nice." I pretty much assume that everyone hates me and talks behind my back, which I don't think would be particularly unwarranted because I'm a really socially awkward person, and my personality sucks. Naturally, I hate the way I look...if I'm ever to show someone a photo of me online (I work online, so my colleagues are in the computer lol), I have to preface it with saying I know I look like crap in the picture. Like, the other day, I was showing a colleague a picture of an event I went to, and I had to say, "Ignore me. I know I look disgusting," and the thing is, I'm sure people think I'm fishing for compliments when I say things like that, but what I'm really doing is trying to beat them to the thought, that way, they won't be thinking, "Wow this ugly chick thinks she's cute." Then if people say I'm pretty, I don't believe it. I know it's kind of rude to assume people are lying to me, but I just don't see myself as attractive and don't think anyone else could either. I don't even believe guys really find me attractive if they blatantly come onto me sexually. This guy I had a huge crush on last semester made this remark to me about my breasts being really nice, and aside from being disgusted as his lack of tact, in my head, I was thinking, "Yeah whatever. You don't find me attractive in any way. You want to have sex with something with a vagina and will say whatever it takes to get to that point." I also don't think I'm really worthy of love...or anything. I don't think anyone cares about me, but I also don't think I really deserve to have anyone care about me because I'm just a piece of crap human. It's a really depressing place to be, but there doesn't seem to be much I can do about it.I don't want to just decide to start liking myself because then I'll just feel like the obnoxious chick who thinks she's entitled to more than she deserves.