The story goes that Chelsy was ready to marry him, to the extent that he asked the Palace to start prepping for the engagement announcement, the security she would need started to be put in place, the usual MI5 digging into her background for reputational skeletons to manage, all those workstreams were underway.
But the sticking point for her was his explosive temper and anger issues; she wanted him to go to therapy and he wouldn't. Apparently it came down to, "It's therapy or I'm off", and he wouldn't, so she was off.
This story was doing the rounds for years before he met Meghan fwiw.
Obviously Kate and William eventually got him to go to anger management therapy, but it doesn't seem to have worked terribly well.
I agree with PPs who say that M brings out the worst in him. I don't think there's anything hugely objectionable about M btw, I mean I don't know her but I've advised any number of moderately successful people in entertainment and they all have that hustle mentality; I admire it. It's so unbelievably hard to gain even a modicum of success in TV, you need to have a measure of talent but you also need massive self-discipline, self-belief and a huge work ethic, so fair pay to M for that. Her problem is that she doesn't seem to have the talent, disposition, creativity, professionalism or mindset to do well at the stratospheric level of celebrity she's now at. And very few do. That's why she's failing at it.
I think that H&M are poorly suited as a couple because they bring out the worst in each other and also seem to indulge in this folie a deux mentality where it's us against the evil world.
Having been in just such a relationship decades ago, when I was still in active addiction, I think it's absolutely possible to love and trust and rely on and adore someone who nevertheless is very bad for you. It rarely ends well.