Some parts of what you wrote about your girlfriend... that could have been written about me when I was in my mid-30s. I was also usually a responsible, fun, happy, positive person. But I could be very unpredictable with a few drinks in me. For some reason, if I happened to get nasty or aggressive when drinking, the main target would be anyone I was romantically involved with at the time.
The difference is I usually had relationships with men who were damaged themselves, so there was a kind of horrific muddling through with lots of fights and drama along the way. I didn't date anyone like you, no-one who would be so kind, loving and thoughtful as to walk my dog and make me breakfast and dinner!
In my mid to late 30s I ended up single. I had a demanding stressful job and devoted my energy to that. I would drink every night alone after work or with friends like that friend of your gf's you've described - who would be happy to bar-hop on a weekday night with me. I put myself in some risky situations with men, and was promiscuous to a degree.
I was not happy. But I was in extreme denial. No-one had ever really called me out on my drinking. No-one had told me to my face that I had a serious problem with drinking - not a boyfriend, nor a boss nor a colleague nor a family member.
I pretended that my drinking didn't have consequences. I ignored that I had likely lost promotions, that it had probably cost me some good friends and promising relationships. I carried on for several more years until I was starting to get sick and failing to function properly at my job. Then I couldn't deny it anymore and finally got sober.
The reason I'm telling you this is that if you have ONE hope of helping this lady, it is not to stick around and be a co-victim of her disease. It is to leave her with no doubt that her drinking has cost her something dear.
Tell her clearly and in no uncertain terms that you are leaving because she has a problem with drinking. That everything else in the relationship was great but you would not put up with her drinking. Leave her nowhere to turn so she knows it is her drinking that is the cause of losing you. And tell her if she doesn't get help it will get worse.
Then leave. Leave, leave, leave. Leave and pray that somewhere in her mind she understands that she screwed up and it was due to her drinking.
I can't say that would have made a difference to me when I was lost in my alcoholism, but on those sad, sorry days when I did wonder if I should stop drinking, it might have led me to recovery sooner to know I'd lost a wonderful man and a happy relationship because of it.