How do i get over psychological trust blocks that i've developed from past relationships where i've had my heart broken?

I just broke up with my G/F for about the 6th or 7th time in the past year. i seem to get to a point where i feel that i’m afraid to “take it to the next level”, and them make up some kind of excuse to break up with her. i’m in therapy now to figure out why i keep on doing this. just a little backround, she is beautiful in my eyes, we have a very satisfying sex life, she understands me better than anyone i’ve ever dated.i guess my question is how do i let down these psyological defence guards that i have inside me so that i can “take it to the next level”?