Mer,
If your health is suffering because you are still with him isn't that telling you something?
Is your therepy working? Is he actively paticipating- is the affair over?
I think if it isn't over maybe he needs to move out until he is either willing to commit to you or accept that you will get a good lawyer and he won't be able to show the young one a good, expensive time no more
Rebuilding a marriage is hard but a half a life is harder and deadly for your health. I am so sorry that you are going thru this
We're all here to support you