Am i worrying to much or something?

Well at school there is a girl that i like. She use to like me a while back but i was kinda stupid back then and messed it up. Now i like her and have for the past year. We have something planned already. Shes going out with someone right now but they have been arguing a lot. I feel like i cant impress her though. I cant tell if im worrying to much or if im just self conchs about seeing her around other guys and it makes me feel like i got competition or something. I don't know if it makes sense but hope someone can tell me why im feeling the way that i am.