In need of advice about opening people up.?

My girlfriend and I have been together for 5 and a half months and she is really bothered by the fact that I know very little about her because I struggle to ask the right questions to get her to open up and talk. I have taken it for granted that if she has a problem that she'll tell me when she is ready. But now she is complaining that other people in her life can read through all her bullcrap and I don't seem to. I do care about her very much to the point I want to be there for her in whatever she's going through. I mean she opens me up so easily but I still close up and it makes her close up to and I realize as well that other people don't know how to approach me with there problems cause I am in a huge sense centred on my own problems. I need help. I want to change this and start being there more for my girlfriend before she leaves me for good. I am a good guy who cares very much for people and I want to show her that I just hope I can learn this soon...