Suggest me a book to help me gain clarity about dealing with a very challenging long-term romantic relationship

I'm looking for a book to help me gain clarity about a my relationship with my boyfriend of four years. Things have gotten very complicated and I am really struggling to figure out if we can make it work or if I need to call it quits. I'd love a book that can help me sort out these feelings and get clarity.

Both nonfiction and fiction are fine!