I can't handle "long" relationships. What do I do?

I'm a junior in high school. I suppose I'm fairly attractive, as hook-ups and relationships are mutual and relatively easy to get into. I used to be super shy around girls. Then in freshman year I dated a girl for 11 months and it was hell. We both had major faults and it ended terribly. I learned my lesson, yada yada. I dated a friend after that for about 4 months but it was nothing serious. Almost like friends with benefits. I couldn't stand the commitment though. I had a summer to mess around and flirt with whoever and now I'm dating another girl. It hasn't been 3 months yet and I'm bored and considering other options. Should I lose hope so quickly? She's really nice and I hate to be a heartbreaker, but we've been arguing a lot and I think this is causing the "flare" to leave. Which sucks. It's almost love/hate. Do you think the bad first relationship is causing this? Insight is greatly appreciated.