He's probably forgotten many conversations.
It's easy to forget the identical circumstances - he sobers up (maybe) for a short while, does all he thinks he should be doing, gets back in to your good graces and it all starts again. Now you are in firehouse management, he's talking about depression and what a loser he is, you are trying to help him and build him up and keep things on an even keel. You've forgotten he was cleaning the kitchen, that's history and you are back in the thick of things, that doesn't leave a lot of time for pondering.
That's why having the list with you all the time is key. If you have a hard copy, put it in your pocket. Every time you get to ruminating about the "good times" or how he needs your help, grab that list, remind yourself where you have been and why you are now where you are with him.
Plus you are used to this life style, you have been doing it for a long time. You seem to have genuine empathy for him. Compassion is much safer, from a distance. For many people wanting to help out someone we care about is a good thing and natural. This is a situation where you have tried and not been successful. It was never going to work, no matter what you did (and I'm sure you have tried to present help to him in a thousand different ways). It's not that he doesn't understand, he probably does, but he either won't or can't change.
You don't really have a reason to feel guilty do you? You didn't do anything with mean intent and you weren't being malicious. You are just looking out for yourself and that's ok. There is selfish and there is a healthy dose of looking after yourself, two vastly different things.