How do I move on from my boyfriend's betrayal of my trust?

Yesterday I found out that my boyfriend of 8 months, whom i live with, has been for over a month now texting another woman he works with daily and going to lunch with her 2-3 times a week. He told me about it, I had no idea it was going on, but her husband threatened he'd e-mail me and let me know if it did not stop. He did e-mail me this morning and sent me a conversation my boyfriend and his wife had over texts and told me everything that his wife told him about it. The conversation wasn't ual at all except there was a comment by my bf that he couldn't get her out of his head and she said the same. He told me nothing physical happened, he knows what he did was wrong, he regrets it and will never do it again. Basically my question is, if I decide to stay in the relationship and we try and work through this, what steps can we take as a couple to mend. Obviously, I have lost all trust in him and am not going to forgive this quickly. I'd like to hear from anyone that this has happened to and what you did. Before this relationship I was with someone for 10 years who continually did this to me and I stayed. I do believe my current boyfriend to be totally different then my ex and he has never betrayed my trust before, but I don't want to be a fool and forgive someone who doesn't deserve it. Although this incident is minor compared to what could have happened had it went on for months, I still was lied to and I know I will have trouble getting over that. I'm just totally at a loss, please help!