@Davidq,
Well, you made most of these problems, yanno.
I say your ex is a saint for giving you the opportunity to move back.
And I think if you've got any kindness in you, you won't do it. Because she's going to be left taking care of a terminal patient who selfishly left her for someone else.
Man up and get professional care (yes, a visiting nurse, etc.) and let your ex and your kids have contact with you, yes, but don't push them into the caregiver role. Because that's where they are going to be. Don't let your loneliness get in the way of doing the right thing by them, and relieving them of this.