why is it so hard to believe for Emma in fairytales, i mean she doesn't even try and it's very anoying!!!!!

Well, at first I think it was just supposed to be because she has spent her life in out world. In our world, if toi try to tell someone that SnowWhite is real, they would probably send toi to a mental institution.
Now though, I think it is not just that she cant ou wont believe in fairytales. Its that if she admits that fairytales are real then she has to accept the fact that the fate of everybody in storybrooke rests in her hands, and that is scary for her.

Think about it from a logical perspective, if someone told toi that toi were a fairytale character along with this whole town and toi were suppose to save them from some curse cast par a Queen, that most people would think that sounds crazy.

Emma had a rough life growing up, believing is a hard thing for her. As an adult, she probably doesn't think toi shouldn't believe in fairytales. Not to mention if she does choose to believe, that would mean she'd have to take on the huge repsonability of saving an entire town which alot to ask of a person. I think part of her wants to but she's plus scared then anything.

Granted it's annoying to us as viewers because we want her to hurry up and believe, but when toi take someone who's had a life like Emma it makes sense.