How to get someone to open up?

so my gf never really opens up about some stuff. like if its something that really bugs her, she wont even talk about it at all. sometimes she gets quiet and distant cuz her mind wanders to destructive thoughts. and she tells me stuff like "i cant; its just how i am; its not that i have a wall built up and dont want too, i just cant" and i know there has to be a way to show her that she can actually verbalize things. but shes stubborn and convinced that no matter what she'll never be able to get the words out of her mouth. and i can see how its destructive to her. it really ruins her mood and brings her down alot. so i wna know how i can get her to speak when she gets all crabby.