Why am I so hard to get?

I’ll explain. Everytime I meet a guy, at first they’ll be like ‘she’s shy’ or she’s classy but after a while they want more and that’s where it ends for me. It’s like I can never give myself to someone. I used to be a lot fatter and now I’m normal I guess and the guy I like,tells me occasionally that I look pretty and people can see he likes me. I have a crush on him but just don’t know how to handle this situation.