I've had 2 major relationship breakdowns.
1st was a marriage, to someone who seemed exciting and interesting and showered me with gifts and flattering attention. 2nd was the long term relationship that occurred after that, with a nice but dull man who, at the time, I thought was wonderfully supportive. It turned out he was just the same drink in a different bottle.
I started by writing something long, describing all of the various harms and deceptions they both did in their different ways that led me to finally reject them outright. In the end, I think I left them both for the same reason.
Despite my help and understanding, neither of them was willing to try to be as good a person as I want to be. I strive to be a good person, and they simply couldn't be bothered. A good relationship is one where you want to be your best self, and be respected and loved by your partner for the investment you make in being a couple. I hoped they would do the same. You can't make another person change, but someone who loves you will be willing to change some things to increase your mutual happiness.
Both of these men were unwilling to accommodate with me in ways that were sometimes small, but sometimes large. They refused to see my point of view if it didn't suit them, and would become sullen and angry. The first nearly drove me into bankruptcy (from a pretty comfortable financial position, which I know he was alert to), mostly due to laziness and entitlement. The second made no secret of his unemployment when we met which I was fine with, but resisted any opportunity to try to gain employment for years. He would frame it as meaning he had less time to support me emotionally, and that a 'shit job' would be demeaning and damage his MH.
The final straw in both cases was realising that they would never care about the actual me as much as I cared about who they had pretended they were to begin with.
Their real selves were far more shitty, but still I tried.
The metric I use now is, would I behave like this with someone I loved? If you wouldn't, you should leave. Willingness to be open to change and compromise shows that you love someone. If they won't, they don't respect you, and will always treat you like a mark.
It took me about 12 years to nail it down, but that's my light eventual light bulb.