Three things that leaped out at me from your posts, OP:
"he's done this three weekends out of five since Christmas"
Seriously? I know you said your idea of normal is screwed, but this is way, way beyond normal. I'd also guess that this is an increased frequency, which means he's getting worse.
"We've been together for six years and have no kids. I've just turned 31, he's 43."
If you think you may want to have children in the future, get out now. You do not want to waste all your fertile years with him.
"We had a discussion where he apologised for not texting me (which is the real issue) and he doesn't want it mentioned again."
He doesn't want it mentioned again
? Who died and made him god? You can mention it as much as you damned well please! I absolutely concur with Rabbit's translation - "Shut the fuck up love. I'll do whatever the fuck I want. Do not say another word about it. Why? Because I said so, bitch. Do as you are fucking told."
This man is showing you no respect at all.
One last thing, I could be way off beam here but when you posted "On paper, my relationship looks like a train wreck" - is there more to the train wreck than his drinking? I just ask because I knew someone whose relationship was very dramatic at the start, lots of problems to overcome, etc. - and when the relationship stabilised and just wasn't working because really they weren't suited - she hesitated to stop the relationship. It was as if, having overcome all these obstacles, she felt she had to stay, almost to justify the dramatic start. Sorry, struggling to put my thoughts into words here, but the way you expressed it made me wonder if there's other stuff going on.