Dh 'why do you want to start an argument?'

[YouthMarket]

Every time I express an opinion that's different from his or means any changes, he says this.

Why? It's infuriating.

No I won't. It would be the end of the discussion if he thought I'd discussed it with anyone, even a close friend or family member.

I think he's v stressed and a bit depressed. I'll go gently.

I think it’s fairly impossible for any of us to make a comment about whether he is stressed or depressed. It’s a valid area to look into as a reason for the change in him.
It could equally be that you have slipped in his emotional world into the flatmate/co-parent role.
I hope you manage to at least start a two-way conversation tonight. I doubt this will be resolved quickly. It’s going to take some time either way.

"He used to be so kind, funny and interested in the world. He's calm and capable and a good dad. There's still some of that ."

He doesn't have to be a bad person, or for you to be a bad person, for the relationship you both had to just not work any more.
If his feelings and behaviour towards you have changed to the point where:
"This doesn't feel like a loving relationship where we want the best for each other. I don't like the dc seeing the total lack of affection. He looks at me with no affection or fondness and almost never laughs. He used to laugh and I miss that."

It's not you being a bitch or starting to shout that hes a monster to decide that you don't want that as the rest of your life.