Oh, honey, I hear you. My story is so similar to yours, it's scary (except the bat sh!t crazy OW still lives right next door!)
I understand him saying that he doesn't want to talk about your x "friend", but this is not about what he wants, it's about what you NEED. You need the truth, and you deserve no less.
Perhaps he didn't see her for what she was, but does he now? If so, he needs to talk to you about that too.
And, honestly, he IS responsible for what happened here. Sure, she was your "friend", but HE was your husband.
The only way to sort this out in your mind is to talk, talk, talk. He says it hurts him, but what about you? You were the one that wasn't given a choice in this situation; he and his AP made all of the choices for you, so, now, he really needs to suck it up and talk as much and as often as you need to. The time-line is also a good idea, it can help to set things straight.
Hopefully, if he is completely truthful, you may one day get past her, but, you need what you need, and if he's serious about healing your marriage, then it's up to him to provide whatever it is that helps you.
Sending you strength honey, and hoping that one day OW becomes nothing more to you than sh!t on your shoe!