So, here's the thing: Whether you're 20 or 40, losing someone you love hurts, and you keep going back and wondering what you could have done differently.
Addiction complicates that. Whatever kind of addiction it is. Addiction to alcohol, addiction to porn, addiction to another person or a relationship.
Right now you're in the worst of it. Al-Anon is a great place to be. Because no matter how much you would have forgiven your ex, and loved her, it would not have made her stop drinking. And trust me, after 20 years married to an alcoholic, I can tell you this: It is no life.
Shellcrusher talked about what you can do in the next 20 years. I often think about what I could have done in the past 20, had I not spent all my time and energy trying to get an alcoholic to quit drinking.
Take care of yourself. Become the man you want to be. That's the biggest gift you can give yourself.