I guess I'm not real good in relationships with a boyfriend and I don't know what's wrong with me...?

I am always playful and cute and whatnot around a guy I like....sometimes shy though and in that case he never even gets to talk to me....I'm sorta afraid of being alone with guys I like. I am almost 15. It's not like I'm like 10 and I am just inexperienced. But once I get into a relationship with a guy I like or even if I don't like him much....I DO NOT KNOW WHAT TO DO. I just don't know how to get comfortable around a guy I like and that likes me back, If he likes me it's cool. Or if I like him it's cool. It's just I don't do well when both.