Red, I've had those same thoughts about my ex fiancé. In fact, she gave me HSV2, and I "chose" to get it from her because I thought we would be together forever. She only had a few slip-ups for about 1.5 years until she couldn't white-knuckle it anymore. The drinking escalated, she got heavy into drugs, and she got so drunk that she kissed another guy and made out with a lesbian...right in front of me.
I clearly told her that her behavior made me leave. I wrote her more than one letter, messaged her on Facebook, sent her a text message, and tried to tell her in person. I worded it every way I possibly could: she can't act like that and be in a relationship with me because that behavior hurts me. Fact is, it's impossible to understand someone that is insane because they do not think rationally. The behavior is narcissistic, and an addict will tell you whatever they have to in order to get you to do what they want. They will tell you they love you and then go out and get drunk. They will tell you they miss you, and they've been messaging other guys for some action. There's a saying that goes: when an addict's llips are moving, they are lying. It's impossible to understand how he thinks, and if you keep trying to do that, you will drive yourself crazy.
I wound up having to completely block my ex fiancé on all fronts because it was too much of a temptation to talk to her and try to "fix" things. Truth be told, even if we get back together, I know that she will relapse because she hasn't gotten into recovery or even gone to counseling. She managed to hold out 1.5 years last time, so how long would it be before she relapses again? It's not worth being with someone that loves their addiction more than you. They will always choose the addiction because that's their coping mechanism. They can't live without it. Every time that you re-engage, you allow yourself to be wounded again. When I talked to her, she said really hurtful things, and when I looked at her Facebook and that of her alcoholic brother's, I saw what she was doing. The wound opened up again every time I went back. I eventually decided just to cut her off completely and never look back. That's the only way to heal. The only way to move on is to stop picking the scab. I know it's hard, but I would rather be alone and healthy than in a relationship with someone that causes me so much pain.