How do I get my gf to talk to me more?

She's a really closed in kind of person. I've told her that she can tell me anything that was on her mind whether it was good or bad, that I'd listen and help her where I can. We've had plenty of arguments and whatnot but, we always find a way to work things out. Most of the time it seems as if she doesn't even want to be talking to me.. And if I question her about any of it or try to talk to her about it she hardly ever responds.. She's messed around on me once, that I know of.. And I forgive her for that, given we haven't met yet and stuff.. But, I'm not gonna go out with some chick because of that.. That's definitely no justifiable reason.. Or reason at all, for that matter.. Just lookin for some opinions about this matter or similar situations or stories. Thanks.