MLS, I can only speak from my experience. First, don't give up. Knowing what I know now, a man would be a fool to let a woman go who wants to keep her marriage the way you do. Stick with it. But also don't hesitate to get tough - you've got to get through to him.
This is what I'd do... and everyone else can feel free to disagree with me. I'd go ahead and get the other woman's number and call her. Seriously. You need to be polite, but firm. It's always possible she has no idea he is still married... and you need to treat her with some respect. I'd simply say, "Are you aware that my husband is still married and living with me? And do you know how hard it is to repair my marriage with you still in the picture? I'm asking you in the most respectful way I know how to back off. Please break off all contact with him out of respect for my marriage. And that includes friendship, however innocent it may be"
Yes, you cheated in the past. So have lots of men and women. But some men will use this as an excuse to do as they please. On one hand they justify it with the fact that you took the first step to destroy mutual trust, and possibly the marriage. On the other, they don't want to be held accountable to standards of behavior they expected you to uphold in the first place.
Then ask your husband to maintain some level of commitment to your marriage. Tell him if he sees her or talks to her again, he has to leave your house and find another place to live while still keeping other women out of the picture. It doesn't mean your marriage is over, but it does mean you are serious. Expect him to ask why you are applying a double standard in light of your own infidelity a few years ago. But that is beside the point. If he's even the least bit smart, he'll already know how much you want to hang on to your marriage and not even go down that road (he deserves a lot of credit if he doesn't bring up that argument).
Just to let you know, I called the other guy when my wife was having an affair in an attempt to get him to back off - and it worked to keep him from traveling to my state to see my wife. When things kept going south and she continued to pursue the affair with him (and I suspect others), I also did what your husband is doing now for a short time until I realized how insanely stupid it was. But I cannot tell you how important it is to take a step to physically intercede in his affair. He needs to know you are dead serious. So does his girlfriend.
Here are a couple of other thoughts. And again, others can feel free to disagree. The trust has been breached a few times, so I see nothing wrong with this. First, collect records of his calls to and from her. Monitor his credit card and checking statements for questionable charges. Watch for secret cell phone accounts, secret email accounts. Install monitoring software on your computer which records keystrokes and sends summaries to your email. If he has any strong Christian friends, one of them nbeeds to hold him accountable. The point I'm trying to make is that sometimes you have to know how far things are going before you know how to do battle to save your marriage. You can still be loving and kind. Obviously God has to take the lead in this battle - but he gave you a brain and a heart and intuition - use them.
It is so hard to learn to trust again. Enough people on this forum know my marriage situation - not good. But it is so much better when you draw lines in the sand. I casught my wife in yet another lie this weekend. Everytime I think there is a chance, one little chance to make it work... the trust is blown. But it's so much easier now that I've established boundaries, so there are no questions about my responses and what I do in turn. We are separated now, I live elsewhere, and we are preparing for divorce proceedings. But I have found that once boundaries are established, it allows me to remain calm and collected if at all possible and react the right way rather than out of bitterness, anger, hurt or even hatred.
Fight the good fight - God's timetable is not always in line with our expectations. He loves you and wants to see your marriage survive and thrive. I hope with all my heart that you are successful. I will be praying for you and your husband.
Southern Cross
p.s. - Love Must Be Tough by Dr. James Dobson is a great book. If you haven't read it yet, pick up a copy or call Focus On The Family - they'll send you a copy.