I suppose i just understand why he either won't get married or end it.
I had this. I was 26, we'd been together since I was 17. To his credit, he wasn't hung up and full of guilt about an ex like your partner seems to be, but he gave the same spiel - waiting for the perfect time, no rush, he'd do it but wanted to decide when, etc.
I posted here; I got the same responses as you. Pages and pages about how he wouldn't ever go through with it. He always talked me round. He always had a reason and, in hindsight, I thought I loved him and didn't want to risk not finding anyone else. The sunk costs fallacy was in force...
I proposed, in the end. He said no. I asked him to leave for a while, he came back after two days and wanted to carry on as we were. We split a few weeks later. I couldn't live the lie; and spent six months rebuilding myself.
I'm with someone new now. We've been together for 2.5 years. The relationship has been a million times better, and easier. There's never been any pressure on either side, we move together. We're newly engaged and he's just as excited as I am.
I can't say that'll happen to you, but I wanted to sympathise that (ex stuff aside), I have been there.