When you learn to NOT let yourself become vulnerable to the WRONG people you'll not be afraid of becoming vulnerable to the right one and that's when you'll not be afraid to commit.
I think your gut is telling you somethng in this current relationship that isn't allowing you to become vulnerable enough to her to let down your guard. If the vibe I'm getting is correct, then all I can recommend is to not ignore your gut.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion