Why do I feel like this and how can I stop it?

You sound like a really great guy, and don't put yourself down just because you get these feelings, trust me a lot of guys in my company are the same. You probably feel this way because you havent had a girlfriend in a while i.e. sex, sometimes men need 'release' if you know what i mean! Just concentrate on getting yourself in shape and on yourself for now and dont get frustrated because you feel that way, just remember that most men, no, all men are the same!