Need help!

[alone]

I have not posted much and in awhile. My husband and I have been back together for over 1 year. I am having a trust issue. I found out on Saturday that when my husband was at a meeting last week for work that the OW was there. He came home on Thursday when I found out about it I confronted him about her being there. He wanted to know how I knew that and I showed him her facebook status showing that she was in the same town as him.
(I know I should not have been looking, but it was one of the things that helped me feel secure when he traveled, is to spy her facebook.) After that he admitted that she was there. He stated that she was there at the meeting to hang out with friends that were at the meeting. (She lives in San Diego)
He said that the group of them had drinks together and that she had just come from Vegas. I asked if he knew she was going to be there and he said no. It really bothers me that he had drinks with them and her and that he talked with her. We talked that he was supposed to go to this meeting in San Diego earlier in the month but did not because he thought she would be there and that I could check with his secretary about him getting out of that meeting.

We talked and he said that he loved me and that he was committed to our marriage. I told him that he needed to be honest about running in to her for our marriage and that when he was not that I feel distrustful and hurt finding out about it. He said that he did not want to hurt me by telling me she was there.

How do I ever get back to trusting, I feel like this has put us so far back and I how do I know that I can trust what he is saying. I want to believe but I don't, I feel that I need proof. Is that wrong of me? Am I supposed to look at how far that we have come and since he says nothing happened leave it at that and suck it up. Even though I am totally upset. I do not want to undo everything that we have done, if this is exactly what he says it was.

I just do not know what to do?