My suggestion is to ignore the "why?" (Sometime in the future they may be able to answer that question, but not now, so it's flat just not worthy of spending any more effort on it) and put your concentration on the "what can I do to make it better and easier for him".

That means you have to see that he gets treatment and does his part even when he doesn't feel like it, plus hold his hand when he needs it plus take care of the kids. If that becomes all too much, consider getting some outside help with one or the other.

Indeed, you do sound like a strong person (and keep in mind that even strong people don't always feel like they are strong!!). Keep coming here for support when you need it because that's an important part -- Folks here have been through both sides of this (victims and caretakers), so we understand.

Lastly, if you really concerned about his survival, then the best way I've found to deal with something like that is to actually formulate some mental plans (don't share them with him, of course, unless things become definitely bad) because dealing with alternatives is often much easier than wondering if the alternatives will become reality (again, separating the stuf we can do something about from the stuf we have no control over).

Hang in there,
Pete