I'm so sorry for the pain you are in. We all can understand it! With this MLC depression, so many of them just feel the need to get "out" because they think their unhappiness is based on the externals (us, their families, etc.) without being able to see it is internal and something they need to look within the fix.
He won't be able to fix until he is ready. It is so hard to hear, I know, and this takes a very long time. They really don't seem to see the damage they are doing (which has been a concept that has blown my mind).
Forever is right. It does get easier and it is a roller coaster that will go up and down. Just try to get your focus off of him and onto you and your girls. That really is the only way. Don't focus on the threat of a D. Have you tried going no contact? Is he living with you or elsewhere?
