What to do now... I feel betrayed?

I just recently learned to let myself trust my boyfriend completely and wholly, thanks to my screwed up past. He is my first love and I do truly love him. There was this girl he had a crush on back when we were just friends. He dropped it, obviously as we started to fall in love. Its been 5 months now, and he danced with her at his father's wedding (I couldn't go. I was out of town). He told me I had nothing to worry about, he wouldn't even look at other girls cause he had an amazing one coming home to him, his words, not mine. I didn't worry. Or wonder. Not at all. For the first time ever. And he dances with her. All that trust is gone. What do I do now?