OP
You say you've made it seem worse than it is.
Before we moved in, dp lived in a hovel. I'm talking something you'd see on TV, it was that bad.
and
I asked if we could clear a shelf
A shelf mind. Just a shelf.
... in the dining room which was loaded with games he'd just left. I started to help and he basically told me to get lost and he'd do it. He ended up throwing a glass award he got from work about 15 years ago into the bin with such force it smashed. I went upstairs, and goodness knows where he went but he went off in the car for a few hours. I went to bed and haven't heard a word since.
Living in squalor for a not insignificant length of time, reaching boiling point to the point of a violent physical reaction, pretty fast, when pressured to clear a small area in a communal living space. Then withdrawal. Punitive, or necessary to simmer down to safe levels of anger? Who knows.
How much worse does it need to be?
Imagine having to tell that story to a social worker in child protection who is checking your children are safe and well.
I'm serious.
Imagining a retelling in that context is a very good reduce my filters, retract my emotional lens, induce reliable reality check strategy. If the very idea of an honest retelling without whitewashing/selective omissions makes toes curl, it's time to de-clog filters.
I know I'm going to get lots of replies saying his behaviour is ingrained.
The downside of support groups, the reason why I left them, is that it forces you to watch people caught in a dance. Where they repeat the same steps over and over. While singing how they have learned new moves, that ... this time will work.
It's not his behaviour being ingrained that worries me. More a concern that your reactions to it might become so too. There is nothing original about your version of this. I don't see the "but this is DIFFERENT !" that you do. Because it is all very familiar territory.
People won't say it is ingrained, and your reactions to it pretty standard to make you feel bad. This is all new, unchartered waters for you, and the current hurl/sell feels like real progress. But the people who point to just how unoriginal the script being written for you is, for them it is just "same old, same old, BTDT, got the T-shirt . A t-shirt that got buried somewhere in the replacement piles of stuff. Replacement piles that were bought with the money (and then some) from a past "ok I'll prove it has value and I am not a lost cause and I can CHANGE !" sell off.
I am not judging you. I have no right to. I understand you. Because in many ways, despite the different kind of relationship, I was you. Besides, I left my little sister in the hoard. We are both in our 40s and I haven't even begun to make a start on forgiving myself for that. I don't get to judge you. I'm far far too busy judging me.
What I can offer you in place of judgment is a litmus test. To help you predict if you will have to spend your entire relationship exerting constant pressure and threats in order to manage continuance of improvements and avoidance of back sliding. (either successfully, or not so much).
Stop the pressure. Act like the storm has passed. Watch where the money from sold off bits goes when there's no prompting, or arm twisting. Without comment. Just observe.
See if he keeps up the clear out off his own back. If this is not a real cause for concern, if it was a relatively, easily fixable "bad habits" sort of thing, that couldn't possibly risk negative ramifications for your children, he'll keep going. Maybe slower than you'd like, but he'll keep going. Because it caused you real pain and drove in a wedge between the pair of you. And the siren song of the hoard won't be able to compete with the thought of causing you pain, or your children emotional harm.
He'll see the improvements as improvements rather than gaping holes of loss. He'll see any ebay monies as minor claw back, not vindication and a green light to buy more. He'll want to add to the improvements, for the kids' sake, for your sake, for his sake. Because, with space, lightness, extra cash... where's the downside ? He'll overcome his blip that became a bit of a habit linked to common or garden laziness. He will come to you for support, help shifting stuff, encouragement and motivation if and when he needs it or wants it during wobbly moments. And you can freely give it.
On the other hand ... if the core issue is a real cause for concern, a not so fixable thing and in fact it could have negative ramifications for your children.. with your foot off the accelerator his forward motion will slow to halt. And backsliding will begin. You'll quite possibly learn to hate the git that bought from him off ebay. Because the small claw back from all the money/time investment will become his magic feather of vindication.
If that happens the question to ask yourself is not, what should I do to maintain his momentum and shunt him off a backsliding trajectory.
The question to ask yourself is ..... do I have the energy to bring up my children to healthy adulthood (with the sort of time/effort/involvement that requires) AND constantly police his relationship with inanimate objects ?