come on, someone has to have a comment??? please

Hi. I'm new here and I never thought my marriage was in trouble or that I would ever be posting things like this to a website. After 36 years my husband had an affair. last year a bunch of things happened in our life. right around the time our son got married we found out that my husband might have prostate cancer. I thought I was being supportive, it sure was a scary things for both of us. I guess I did not show enough care (with wedding planning and all) and he started texting a woman that had been his 9th grade girlfriend-found her on facebook. Anyway, she lives 1300 miles from us, so them getting together was not easy, but she did fly here one time and join him when I thought he was at a overnight meeting. That was last Oct. The texting and cell phone calls continued until March. By then, I had gotten sick of him ignoring me and working all the time. He works from home so it was very easy for him to work all day, eat dinner with me, and then go back in his office until 9 or 10, and then go to sleep. We really didn't fight - never did -but any communication we had was sarcastic and just pushed both of us appart. I approached him with "what has happened to us" and that is when he told me that he had been talking to this girl, had seen her once, and that he did not love me anymore. I knew something was wrong, but I really did not expect THAT. He said he needed someone to talk to and he did not feel like I cared.
I was in a panic, it was right before our anniversary and he was going to be away on another "business trip". I asked him if she was joining him and he told me yes. I said that I wanted to go with him instead on his trip, that I loved him and he was the best thing that ever happened to me, the only person that I had ever loved (since I met him when I was 15) and to my surprise he said OK. He called her and told her not to come and I went with him instead.
We are now seeing a counselor and he has broken all ties to the other woman (or so he says)We are communicating and sleeping together again.
 My problem is the trust and forgiveness. He had never given me a reason not to trust him and forgiving just seems impossible. To me,saying that I forgive him makes it sound that it was OK. There are days now that I just have these obsessive thoughts about him and her together and I wonder if I acted to quickly telling him that I wanted to work on our marriage. Does this see-saw of emotions ever end? How do I make sure he really is not talking to her anymore? The doubt in my mind some days just makes me nuts. One day I think we are doing pretty good, and the next day I'm talking to myself saying "he had an affair - are you nuts for putting up with that - why am I still here?"
we are only into the "repair" part of this thing for 3 months. How do people learn to FORGIVE
Any thoughts would be appreciated