I know this is painful but you were not together. Some people would say you have a right to be angry because you were still together (me included) and some people would say you are still separated. I guess it depends on how you look at it. That is up to you. I think he should have honored you are your vows and not started dating. What I really don't understand is why he is still commununicating with her. They are not friends. They were lovers for one date. It serves no purpose to continue to talk to her. What is his purpose. I would want to know that and I would not be working on things with them communicating.
This situation could definately benefit from some marraige counseling. Whatever the issues are you may not be able to resolve them on your own. A counselor could help isolate the issues so you are not just taking a break and then reuniting with the same issues. Take this solution up with your husband and tell him you want help. You could also talk about this date.
If this has been helpful press accept