I, too, was married for 20 years to an alcoholic. I, too, should have left years before I finally did. Mine was my best friend and worst enemy, too. And, mine also cried and tried to make up for it after it was too late.
Here's how I did it. I gave myself six months of living apart before I would let myself consider whether I wanted to remain married. (It actually took more like a year and he did get sober and we did try to reconcile.) I told myself that if it was meant to be, it would work out after some time. But, I could not go on living like we were. It wasn't about giving him another chance, it was about giving me some space and peace. If he changes into the man you actually want to be married to, you can go back. But, right now you have to to what's best for you right now.
It worked for me...
L