Dating GF who cheated

I'm only 18 going on 19 in september.

my girlfriend is about the same age. I love her to death... i really do. but loving her has been an epic battle in my mind since day one.

it all started 4 years ago. I liked her, and she liked me... and she liked someone else. she often chose him over me (she even admitted it was for sexual purposes, because i didn't want to do anything sexual and he did.)
finally after about 3 months of me loving her and her lusting him she realised she needed to set her heart first.
She ended up dating me. the first 6 months we never had a single argument. from there it was a downhill slope. Eventually i gave into the sex thing she wanted (after about a year of us dating). which helped for a while but then the slope continued downward. i started thinking she was the craziest, most pessimistic, and jealous girl on planet earth. after about 2 and a half years of dating i was told she cheated on me with some guy i was rather suspicious of in the first place.

So i broke up with her. she nearly immediatly dated him. which lasted about a month. I stayed broken up with her for almost a year. (the entire time she claims to not have done anything sexual with any of the guys she was with at the time. even though i know her to be quite an aroused girl and i know she drank alot and did some cocaine during this time, and also got naked in a hot tub with some girls and guys). now you are all probably thinking she is a horrible person. and im NOT saying she isn't. but eventually she broke up with everyone stopped doing drugs and didn't drink nearly as much, basically because she wanted to look good for me (so she says). Now... the entire time we weren't dating she says she didn't cheat on me in the first place.

I finally gave in. i am dating her agian. she doesn't do drugs. she hardly drinks, she's very clingy (in a good way) but honestly i dont know what to beleive about her. DID SHE CHEAT ON ME!? and im a good person. i dont do drugs (other than the occasional marijuana), i dont drink, i'd never cheat on anyone. and she's starting to become overly jealous and difficult to handle at times. I worry her negativity will one day make me negative too. it hasn't yet BUT SHOULD I WORRY IT WILL? I love the little things she does when we are together, like the innocent cute faces she makes and whatnot. just little things that seemed to haunt me the entire 11 months we weren't dating. i feel like i cant be without that. but she's such a bad/angry person. What should i do? stay with her, conciderring she's alot better than she used to be and she quit doing so many things she liked for me. and spent most those 11 months we werent together trying to win me back. or should i end things because she is still ruining my happiness and often stopping me from even hanging out with guy friends or having fun when i clearly should be... should i?

someone needs to tell me something. i just need someone else's advice. that's all.