How can I get my girlfriend to feel more comfortable expressing her feelings?

Trust. You cannot "get" your girlfriend to feel more comfortable, but you can help create a healthy atmosphere for her to share in. It could be easy to slip into a control battle with this, but i hope you won't. Offering your feelings, asking about hers, and genuinely being open to what she says may allow her to trust that you'll honor and accept her as she is. Good luck.
Expressing feeling? Try to be open with each other and encourage her to express her feeling always.
Talk. Tell her how you feel, and focus on your feelings, not on her actions, or she may get defensive or just shut down. Maybe you feel confused, unappreciated, or abandoned when she doesn't give you feedback on certain things and you want to feel more connected. Tell her you want to take the relationship to a higher level and you want to really know more about her, including her feelings.