Need advice from those with years with this to give perspective

I have been reading a lot on this forum lately, finished Melissa's book and have been to two therapists for myself.  I posted the issue about my husband's difficulty with parenting tasks and his possible ADHD.  I've tried to get him to his doctor to get evaluated (won't go) and tried to get us into marital counselling (he keeps telling me"Let's just try this and if it doesn't work we'll go", but I know he is very resistant to going).  The problems are definitely starting to affect our 3 year old.

I know my next/last move is to give him the ultimatum that he/we get some help or the kids and I leave.  But I am so scared to do that because I'm worried that leaving might be worse for the kids than staying.  I can't imagine how he'll parent on his own and I know he will fight for some sort of custody as a pride thing.  The two big realizations I've had from the reading I've done are that I can only help me, I can't change him and that denial is the big relationship destroyer - not the ADHD itself.  But to help me, I need help communicating with him and he won't agree to get help - this is so tough.

I'm so impressed by the ADHD posters on this forum - ADHDmomof2 and JJamieson in particular I've been reading your posts and am so impressed at how much effort and self-reflection you have to go through to make things easier for you and those around you with the ADHD.  I just don't see my husband ever being able to put in that amount of effort.  I've also read the posts of some of those who have lived with ADHD spouses for years and have made sacrifices above and beyond what I'm willing to do and I just don't know how you do it.  

So for all of you, I'm wondering - is there any advice you would give someone whose partner is clearly in denial but who has very young children (3 years old, 8 months old)?  Is separation better?  Is waiting better.  Any help is appreciated!