I am a kind and gentle human being and have opened up my heart to help, comfort and support her as much as i possibly can.
I was a kind and gentle human being who tried to love an alcoholic out of his drinking and out of his demons. 20 years later, my career, my health, my confidence, and my finances were ruined. And he was drinking more and fighting more demons.
I love her deeply and suffer her pain with her.
That's your problem right there. If you are suffering her pain with her, you are enmeshed in her person. When you are no longer able to say "My girlfriend is really anxious about being around her family. I feel compassion for her." but say "My girlfriend is really anxious and therefore I am too" you are in deep doodoo, my friend.
I know you probably find many of our post harsh and unfeeling. But I do agree with Katiekate. About the romanticism etc. What is the story you're telling yourself? It sounds a bit like a classic fairy tale, where you take the role of the person riding in on your white steed to save the damsel in distress. No, I'm not poking fun at you because that's what I did, too. Except he wasn't a damsel but still. AXH was hurt by a cruddy childhood and I was going to love that out of him. My love was going to make him happy.
Doesn't work. Sorry.
As for cheating. Here's my thing and you may or may not agree:
There is not enough alcohol and drugs in the world that can make me cheat on a person I truly love. Yes, alcohol affects your judgment. For sure. However -- it doesn't change your personality, it only brings out what you've tried to hide.
If she cheats drunk, she can cheat sober.