Yeah. What I found the most helpful was The Gifts of Acceptance: Embracing People And Things as They Are by Daniel Aaron Miller (Amazon link).
I'm so glad my comments are helpful. I think it's simply just because we are going through similar life challenges. I read this type of book before I go to sleep. It's pretty good for preventing the "crying myself to sleep" scenario because it puts my mind kind of in the right place.
Another thing that helped me was I wrote a letter to him and a letter to myself. I wrote all the things I wished the relationship would be. The struggles we had, the happiness we had. Then I wish him well, and I wish me well. When I do this I usually start out crying like crazy, but it's really good for getting it out, eventually stopping the tears and feel better afterward. I realize I can help him and myself by letting him go.
Other books I found helpful include:
And some TED Talks and useful YouTube videos that helped me:
Good luck!