I went through a very similar situation. Luckily, eventually I got an uncontested divorce so it wasn't drawn out. I had the same emotional reaction- I begged him to stay when he left, even though the relationship was destroying me on many levels. I spent many months alone, crying and second guessing myself before I filed for divorce. I blamed myself for not being able to make him happy, or so I thought (would make him stay). What I didn't realize at the time was that it wasn't about me. Looking back now, it was the best decision that I ever made. But it was also was the hardest.
It's called a break up because something is broken. You can spend years trying to change him, but the only thing that will change are the years you lose trying. It will wear you down and exhaust you in the process.
I think you've gotten a lot of good advice and support here, but in the end only you can chose to stay or go.
I wish you strength as I have walked in your shoes and it isn't easy. ((Hugs))