Why do I feel guilty/bad talking to other girls, when I have no reason too?

There is a girl I really like.. she has been seeing the same guy for almost two years. They hvant really been getting along lately and have kind of been hoping I could have a chance but they're still together. This girl is perfect for me and I really want her to be mine. But I know its just not going to happen right now. sometimes I even feel worthless when I see my friends trying to flirt with her. but the weird thing is since I know I don't have a chance right now, I have turned to flirting with other girls. every time I do though I feel sad. or even talk to them sometimes, I will feel sad and pissed off at myself as if I feel I have gone wrong. Why do I feel this way ?
Why do I feel guilty/bad talking to other girls, when I have no reason too?
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