Why do I allow depressive thoughts come into my head, how can I block them out(if that is even possible)?

[Chris]

I'll try and make this short and quick as well as informative.

Ok, so I like this girl, we have spoken a phew times, she is in my class at college but she is not in my social group, she is in another. In this group there is this guy who, by my perspective, seems to like her, he always makes her laugh and try's to get close to her whenever he can. Basically this plays havoc with my mind of thought, I always look back at when I see her laughing at whatever the guy said and always imagine her going for him instead of me. This thought really brings me down and it can be depressing. The only good thing I can take from this girl is that she looks over at me quite a lot, it was agreed by a lot of people that her looking is a good sign. Anyway, how can I get these thoughts out of my head, it's lowering my confidence and no way can I stand a chance with low confidence. Thanks.
@Kayla: Are you on my profile to see how many times I ask this question? I am asking it quite a phew times to get as many answers.