Anxiety When With Girlfriend

So I've been with this girl who is amazing and everything I could ever ask for, and we've been together for over 9 months. However, whenever I am with her, I get an immediate feeling of anxiety. Its gotten so bad, that I've broken up with her and pushed her away from me to see if this was indeed my problem and that she was causing it. She know all about this problem, and tries so hard to help, yet thinks she is doing something wrong. Every time I'm with her I'm usually okay for a little but then again I get a rush of anxiety. I even get it when I think about us hanging out. I want to be able to have fun with her and do various things that couples do, but all I ever wanna do is lie down when with her and moap and watch tv/do nothing. It's such a horrible way for me to live, and its been ongoing for months. Ive seen doctors and they have put me on Lexapro but it didn't help at all. I want to be with her and am trying to hard to get rid of this anxiety. Please post if you have any ideas on how to help with this, for it is consuming my life.