I once dated a girl who had an abnormal psychology degree and helped out with mental patients during the day in the extent that she reviewed their daily activities and had to jot down their behaviors. When we were dating she would try to give me a prognosis for everything I'd say. She was also a bit of a nympo too. Which wasn't too cool because she'd always want to talk about previous experiences.
The best thing you can do is be straight-forward with her. If she's a psychologist she's gonna filter through all the bull shit.
My ex who was the psychologist broke up with me and told me I wasn't there for her when she would tell me that I need to keep her thinking positive and keep her mind off of work when we were together. Turned out, her breaking up with me was all about her trying to see if I'd come crawling back. I didn't, and I felt a lot better because a few months down the road, she told me "I'd really like to get back together, I had a lot of fun times with you".
I told her, "Nope", and the relationship ended then and there. Shortly thereafter, I saw her with another guy she had by the balls.
I'm not saying that is what your girlfriend is like, but just be honest with her and say you're not really into this relationship anymore.