Why do guys always choose other girls over me?

I have low self esteem because they tell me how beautiful I am, how great of a person I am, etc but they always choose someone else over me. I always end up being "just friends" with them. A lot of them Lose feelings for me after a week up to a month of being with me. I don't give up sex easily and the guys I date are willing to wait for me, but they lose feelings.

Is there something wrong with me? What am I doing wrong? I do all I can. I support the guy on whatever he wants to do, I don't take away his freedom like most girls my age do, I get a little jealous sometimes, and when we go out on a date I tend to dress to impress him.
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By "give up sex" I mean that I don't give it to them right away. I'm still a virgin. They guys say that that's good and they're proud of me because they're tired of dealing with girls who only want sex.
Why do guys always choose other girls over me?
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