M.W.
Love is a choice. You must remember that.
Have you cut off all contact with this other man? Because it is just human nature to want things we can not and should not have. So, you need to remove the temptation.
Go to a Pastor and ask him to counsel you (and maybe you and your fiance' together.) Christian counseling has helped some people that I know -- who were not Christians at that time..... They also went to regular counseling and they said it did more damage than anything.
You should watch the movie FIREPROOF and do "The Love Dare" on Nick. You can get it from your local library or a bookstore. (This movie is also a good movie for couples to watch together but since he is already making an effort I suggest that you watch it by yourself first and do (or at least start) The Love Dare on him BEFORE you watch it as a couple.)