Should I just talk to her less and spend less time with her?

[Zoso]

There is a girl I like, I had asked her out about 4 months ago when I found out she had broken up with her bf. She told me she 'wasn't looking for a relationship'. Which was just code for she was already seeing another guy. Long story short I knew she was seeing someone else but I wasn't sure how serious it was so I always kept the possibility of being with her in the back of my head because I really like her a lot and we have such compatible personalities. She never really talked about him that much and she always calls him her 'friend'. Well this weekend she suggested we both go together to see a Led Zeppelin tribute band (we are both die hard fans). I guess I saw this for more than it was. We went to dinner before the concert and that night was the most fun I had had in a long time. Then as we were leaving I was gong to take her home then she had her 'friend' pick her up at the venue. I felt really stupid after wards and I don't even really want to talk to her right now. It seems like once again I have put myself in a position where I have become just like one of her 'girlfriends'. This has happened to me more times than I want to remember.