hi everyone! newbie here....could this be a MLC?

[MrBond]

"Could something from 3 years ago trigger MLC now if he has never really processed his feelings about this?"

Again, this is not MLC. It's just unresolved issues. But that's his job to work out, not yours.

"To be honest, I’m not entirely sure what our hot button arguments are aside from having children (which up to a year ago, he had been on board with). His other complaints include that I’m too messy, too clingy, insecure, nag too much, etc. I’ve been making progress in all of these areas, but have not seen an abatement in his anger/resentment, which kind of makes me wonder if his anger/resentment at me is a mask for other issues? "

Make the necessary changes and for the ones that affect him directly, show them to him in small doses. If you seemingly change overnight, he won't believe your changes are real. But if you make them small enough that he doesn't even realize you're changing for the better,the changes will seem much more real.

But regardless of what, if he says alot of things that are hurtful to you, walk away.