It sounds like he's using your mistake as an excuse. You're right when you say that in the face of a 13 year relationship, one slip should be forgiveable. He's going for the other girl, and he doesn't want you in the way, so he's blowing this out of proportion to make you into the bad guy.

Tell him to call a spade a spade. If he wants to leave you for someone else, he should at least have the guts to be honest about it instead of blaming you.