Hey Atom. First off, great job on working on gettin yourself healthy!
I'm 15, too, and believe me I know how easy it can be to, umm, think with the wrong head. I've gotta tell you though, there are things about a relationship that are WAY more important than sex. If I'm counting right, when this all happened you were 13 and your cousin was 12. That's just when the hormones are revving into high gear, lol. From the way it sounds to me, you aren't wanting a real relationship with this girl, only something that you haven't managed to get from anyone else.
My advice would be to stop trying to get sex from girls and concentrate more on building a good relationship. No matter if the girl is your cousin or a girl from your school.
Also, would you please refrain from using bad language? This ain't a locker room and there are ladies present.
Good luck, Dude.