Dear Holdtight
This is my first post on MN, which I am doing to try to help a little perhaps by giving you a slightly different perspective on what you are going through, even though this may provoke a barrage of MN comment, as I am in a very similar position to your DH.
I am extraordinarily lucky that my DW has stuck with me, despite my having had an affair last autumn. Having seen the results of this over the 3 months since she found out just before Xmas, I can only imagine how difficult it must have been for you, with a second huge blow when you found out that he has not been honest on NC. I make no excuses for any affairs starting, but offer information that I hope will help you to decide what to do next.
Firstly, my own experience is that honesty and truth are the only foundations on which you can rebuild your relationship. Some of the other posters have said the same, and it sounds like he still cares for the OW (as I do), but that he is trying to keep you both happy with all the deceit that involves. That does not mean that your relationship is on the rocks, but he must stop the lying if you are to succeed together. Once in a pattern of little lies, it is shockingly hard to avoid more I know this from experience, as I did not maintain NC, and broke my oath to my DW in doing so. This caused her a lot more pain, and made the mountain higher and steeper that I will be climbing for a long time to come. I think that you need to talk to him and tell him that there are no more chances he must come completely clean, be 100% loyal to you, or he must leave.
Secondly, (and as you have no doubt already started), you have got to get to the bottom of why he started the affair. It sounds trite, but if you were both totally happy, then he does not sound like the type to stray (I certainly wasnt). The honesty part of this is again the key; my DW and I discovered that there were all sorts of problems and half-truths that we not revealed in over 20 years of knowing each other. In order to make your marriage work, you have to address the underlying problem(s), whatever they might be. Relate might help here, and just talking about everything evisceratingly honestly will be painful to start with, but it certainly has helped mw DW and I to understand each other much better. It takes time to change patterns of behaviour and he must also be honest with you if/when he is struggling. I thought that I could manage this overnight, but it is still work in progress.
I hope that you succeed and that he understands just how lucky he is to have you still there perhaps he should read this thread to understand things better? Divorce is hugely damaging to everyone (and children especially) so my thoughts are with you both good luck.