I've never really had any doubts about whether or not my Girlfriend is 'into' me. However, I have noticed that my Girlfriend seems to have doubts about whether or not I'm into her! I am, of course, but she doesn't always believe me.
Due to a previous bad relationship, she's not exactly comfortable with the 'trust' aspect of relationships. She's getting there, though, and I try my best to show her love, etc.
Women are so complicated, but that's what makes them so great.