There is a video on youtube of a man taking video of a train as another train approaches at high speed unbeknownst to him as he is, literally, standing right next to the tracks. He is completely unaware that he is about to die. The train misses him, no kidding, by inches.
That is what just happenned to you.
There are other vidoes of people, equally unaware, being hit by that train. That is what happenned to many of us here at SR, including me. I haven't died yet, and I've been trying to recover for seven years.
Having said all that, I'm in Al-Anon and believe there is much to be learned there, whether you are currently dating an alcoholic or not. You can find a meeting here:
How to find a meeting in the US/Canada/Puerto Rico
Lastly, you said,
"I also feel like this wouldn't have happened, and he would have had an easier time not drinking, if I had somehow been more supportive and a better girlfriend." You also said,
"I can't fight the feeling that clearly I've failed him, and didn't understand him or his problem well enough to support him, and the responsibility is as much mine as it is his."
Not only are these thoughts and feelings exactly the reason to go to several Al-Anon meetings to see if it can be helpful to you, it is also exactly the opposite of the truth. You being supportive and a better girlfriend made it easier for him to drink, not harder, but he nevertheless is 100 percent responsible for his problem. He is an adult alcoholic. You are zero percent responsible. We marry to have life partners and/or to raise children together, not to be married to a child. You were about to marry a child, and would have had to care for him in that way.
Take what you want and leave the rest,
Cyranoak