Hugs ((((((Whyme))))). Your are going through the hardest part right now. It doesn't seem like it right now, but it will get better from here. You said you told him from day one you had no intention of being a single mom. You might as well have told him you fully intended to travel to Mars, because it wouldn't have made any difference. He was going to do what he wanted to do. I don't blame you for hating him.
You have a great deal on your plate with four disabled kids. I suspect he stayed 20 years because he needed a caretaker for his kids. But now that the decision is made, you should remind yourself that your kids have two parents. Insist that an equal share of their care falls on him, both financially and physically.
I saw a perfect example of this once with someone I worked with. He was married with 6 kids. He was very handsome and had a high paying Wall Street type job and dealt a lot with other securities firms. He was well aware of his attributes and had at least one affair with a co-worker that I knew of. Then he went on vacation for two weeks and came back all tan. It turned out he was with the daughter of the owner of another firm, on an island vacation while his wife was taking care of 6 kids. He saw his opportunity to trade up and marry into a very wealthy family well connected in the financial world, and he did.
His ex wife got the house because she needed the space for all the kids. He and his new love moved into a luxury condo with only 2, maybe three bedrooms in a brand new high-rise condo building with amenities galore. He saw his 6 kids every other weekend, and just sent them down to the pool all day. He was fun dad.
Then his wife met the man of her dreams. He was honest, decent and extremely well off. They got married. Shortly after that she showed up at his condo unannounced with the kids in tow and a great deal of suitcases. Her and her new husband were going on a 6 month trip around the world and she told him it was his turn to take care of the kids, and she left. What could he say? It's not fair? Suddenly his love nest in the sky wasn't so awesome with 6 kids in 2 bedrooms. Seeing that taught me that women, and men, all too often take on way more than our share when it comes to raising the kids. When our spouse leaves we automatically say "Now I'm a single mom or dad." We aren't really, unless your cheating spouse is dead.
You have every right to expect that he do his share. It doesn't mean you don't love your children, or you don't care because you need a break yourself. Just remember, it's not like he shouldn't expect to do his share. You warned him long ago that you had no intention of being a single mom. So don't be.