Hey Ipcon
This is a really difficult situation you are in. There seems to be a common theme for so many on this site of trying to help a loved one who does not seem to be accepting of help. Since you are the one asking this question I want to focus on you. I am sure you have heard the old adage that you can't ever change someone else...the only person you can change is yourself.
What do you ultimately want from your relationship? What is your bottom line? What will you tolerate and what will you not be able to live with? What boundaries do you have for self preservation?
The only person who knows the answers to these questions is you.
Does he know how you are feeling? Have you told him that you are considering leaving and why? I would give him that chance, at least, to know your present state of mind. I wouldn't deliver it as some angry ultimatum but as a matter of fact portrayal of your feelings right now.
There is nothing easy about your situation. What does your heart and gut say? Always trust your gut.
Let us know what happens. I am hoping others can share their thoughts and experiences. I wish you the best of luck.