Go here:
http://www.thework.com/pdf/JYN_4QBW_10_06.pdf
Fill out this worksheet about your girlfriend then ask yourself the 4 questions at the end.
For instructions on how to do The Work you can read through this:
The Work of Byron Katie
It's about awareness of the thoughts that are causing you such turmoil. It really will help - I've done this process numerous times.