True Colours

Have any of you guys ever seen someone for who they really are after a long time of thinking differently?
Or thought the world of someone, only to find out that they were stabbing you in the back the whole time?

I recently found out that my girlfriend has been lining herself up with another guy while stringing me along.

Let me explain.

Me and my girlfriend had had issues in the past, but we always got over them at the end of the day, I always looked past her problems, and she sort of looked past mine. For example, she kissed a guy at a party, and I forgave her for it. Recently, I did the same back to her by kissing a girl while drunk and having just been mugged. Its a long story, but I had my reasons. This put our relationship on the rocks. For a week solid now she has been telling me that she needs time to think if she can take me back, and I've jumped at every word she said, phoning her when she wanted me, and running round her house when she needed me. Yesterday, she left me, with no reason, just saying it was for the best. I have now found out today that the whole week she has been saying she needed time, she's just been talking to another guy, making sure he'll have her if she leaves me.

It actually crushes me to think that I put all my trust and faith in this person, just for her to throw it back in my face. She made me believe she was the best thing since sliced bread, and that she would never do anything to hurt me, when she was doing it all along.

I just wanted to know if any of you have had a similar experience, and how you dealt with it if you have.

Thanks.