Teen relationship help?

Ok well ive had this girlfriend for 15 months and its been a crazy 15 months. Its always her way or no way. And its frustrating. We fight almost all the time. But when we have our good times there amazing. And finally i just snapped. Im 17 years old but yea i was drinkin at my friends house and i got pretty drunk and kissed another girl. And i feel like **** about it, you dont have to tell me how much of an i am. Now she uses that against me when anything ever happens. Which i guess she has a right to but its gotten pretty bad. Ive tried to end things before but she makes me feel bad and we end up getting back together. I am kinda on the edge of breaking up with her for good. But everytime i think about it i just think about her being with another guy, ually and not. And that drives me crazy. That may show my teen side a lot i know. But i just need some advice. please help?