Why am i so shy around guys?

I hate how I can talk to guys I like on msn & be able to joke around & be myself, but then when I see them at school I just freeze. I've always been shy around people in general, but around guys I turn into mush!! sometimes I cant even talk! I feel so stupid! All I do is smile and try to act flirty but I only seem coy. I usually end up getting ditched by guys cuz I'm so shy & idk what to talk about & they think I'm boring so they go off with some skank who isn't shy at all. I hate when that happens cuz it just makes me feel worthless and it makes me think that no guy will ever want me just because I'm shy & I don't feel comfortable doing certain things. Will I ever find a nice guy who respects me & isn't just out for one thing?