Honestly, OP, I think it's troubling that he "blamed" you for "having" to send her an email setting out boundaries. As an engaged man, he should have been determined to do that from respect for you. Similarly, his snitty asking if there was anyone else he needed to cut out does rather imply he saw nothing wrong with sustaining a relationship in which someone behaved as she did towards him.
Having said that, you know him, you know the relationship, and the choices must, obviously, been yours.
Isn't that what marriage is about, growing and learning through good and bad times, not jumping as soon as things become uncomfortable?
In a good marriage, where it's reciprocal and mutual and both of you try hard to do that for one another, yes. That's the ideal. But I would gently say that in your relationship, you have endured 18 months of hurt so severe you needed counselling over this woman, and he is only now agreeing to back away because you have given him an ultimatum and he is afraid to lose you. If he is genuinely aware of the enormity of the burden he placed on you, and sorry and regretful and determined to change his approach to relationships that may (this was an "is guaranteed to" but there will, in future, be "may"s) threaten your marriage then yes, great. But so far, all the work and attempts to learn and grow seem to have come from you, in an attempt to understand him and meet his needs. And that is sadly common, in marriages - one person servicing the other to the point of being a doormat - but it's not what marriage is all about IMO. The opposite.
What I'm saying is that he owes you a lot of support and reassurance and solidity from now on, because there is a lot of damaged trust and at best emotional laziness, and worst selfishness, to compensate you for. He's hurt you a lot. Swallowing that hurt and reframing it as mutual growth isn't really that ideal, I don't think - the person who needs to be doing the growing and having the counselling sounds like your fiance.