OP, I have BTDT and it gets better!!!! Don't worry!!!!
I moved into DH's house when we got married about 2 years ago. Since then, it has been a daily, constant struggle to eke out a little bit of space in the house. But every little step makes it better, and easier. It was worst when I was pregnant and emotional, because we would get into arguments like why-does-he-get-two-and-a-half-closets-while-I-get-the-leftover-half, and I would just lose it and start sobbing, "I will never be able to live here! This will never be my space!" It was awful. So I totally know how you're feeling!
Only now, 2 years later, after daily small steps of claiming space and cleaning things out, I finally a) know where everything is in the house, and b) can maintain things as they are. This means that if he shoves junk into one of his old junk-collecting spots, it won't stay there -- it has a home somewhere in the house, and I can either put it there myself or make him put it there.
Here are some things that made a REALLY BIG DIFFERENCE for us:
- My parents (aka, my co-conspirators) came over when he wasn't home and helped organize the basement. They brought over two big floor-to-ceiling shelving units, and we just packed up a lot of his excess junk in boxes and stacked it on the shelves. This made room for a TON of my stuff to be moved in. He's never noticed that any of that stuff is missing.
- We needed our one extra bedroom (which had been his junk room) for the baby. So we were forced to clean that one out. I wouldn't let him "migrate" the junk to somewhere else; he had to get rid of it. Except for a few boxes which are still sitting in the basement. But their days are numbered... So my advice to you is, come up with "necessary" uses for every room and storage space in the house.
- My DH is really frugal, so when I start suggesting that we might need to rent a storage space to deal with our junk, that lights a fire under him to get rid of stuff.
- We have loads of sentimental stuff too, from both of our grandmothers, and we struggle with it all the time. What I do is try to use certain things regularly (like his grandma's pretty painted plates, or my grandma's glass salad dressing ladle) so that they feel like they're "used." Then, over time, we'll be able to get rid of some of the extra stuff we're not using, even if it's sentimental.
- I keep the house clean and the surfaces uncluttered. He loves it. This can also help motivate him: "hey, don't you love the way it looks in this room? Wouldn't it be nice if that room could look like this?"
It's a slow process, but it feels like "my" house now -- it took 2 years to get there. Don't give up hope!