I can totally relate. It's really hard to de-clutter when the other half isn't willing to pitch in.
I agree with pp's that the only thing you can do is continue to de-clutter everything but their stuff and then hopefully they will see the light.
We live in a tiny house with 2 bedrooms, one bedroom is ours and the other is kid's bedroom/office. The office is the most overwhelmingly cluttered area in our house, to the point that you can't really sit comfortably at the computer because there is no place to put your feet under the desk, but you cant move the chair back because there is too much stuff in the way.
It's mostly dh's domain, but since there is no room that is only dd's, I have been on his case a little to clean it up some more so she can have a better space. He has a super hard time with the idea b/c "it's his only personal space". Which I have a hard time with, because I don't actually have any
personal space and he also has a jam-packed full of stuff band room in the garage.
Anyway, I just continue to clear out the rest of the house, have our 3 yard sales every spring summer and fall every year and freecycle like crazy.
Eventually it will all work out, hopefully sooner rather than later.
So, anyway, I feel ya sister!