PMS-ing...

These feelings have been building up over the past few weeks and I really need to let them out before I go crazy...

Where do I start?
I'm 19, almost 20...and I haven't been with someone in almost 2 years.
I have only been with 2 guys, both of which screwed me over
My last boyfriend wasn't very nice to say in the least. He pretended to love me and kept pressuring me to have sex with him. When I refused to, he started acting weird, and I have a feeling that he was cheating on me too.
I know it's been almost 2 years but I still feel really stupid and worthless and hurt. I just keep wondering what's wrong with me. I thought I had completely gotten over it, but today randomly, I saw a picture of him and the memories came back and I started crying, and all those shitty feelings came back.

Which brings me to now...
There's this one guy, who I really get on well with. He teases me, I tease him back. We have good banter. I quite like him, he seems like a genuine nice guy. But I have this feeling of sadness in me because I feel as if I'm not nice enough, pretty enough. I keep acting really stupid around him, I say stupid stuff, and we talk quite a lot online, so I feel as if I talk too much to him.

There's this other guy, who I also get on well with, not as much as the first guy, but he's pretty nice and really good-looking. My problem is that I just feel so completely stupid around him, like he's this really cool person and stuff, and I feel unattractive and insecure, even if he did say I was "nice-looking"...
My friend likes him too (I think) and she talks to him a lot more, and compared to her I feel very inadequate. She's sexier, more confident, outgoing and less crazy, she's more out there than I am and I feel like he'd (as well as most other guys) probably prefer her to me. It just makes me feel sad that I can't be as confident and as cool as her.
What's wrong with me?

Why is it that in order for someone not to run away, you have to have sex with them quite quickly?

Thanks for reading.