How to make an ultimatum not sound like an ultimatum?

So, we are at the point where I’ve literally done all the research, said everything 500 times, and made my feelings very clear.  My husband’s anger is, at times, explosive and that’s just not ok.  Even if it’s infrequent.  I’ve used “I” statements, soft starts (per Melissa), and tiptoed carefully around him with my words, which often get twisted and misinterpreted anyway.  I’ve made suggestions about meeting with his doctor, changing meds, and trying alternative therapies, like coaching.  I’ve recommended getting his sleep and diet back on track, exercising, etc.  

But, at some point, after you’ve done all that REPEATEDLY and he hasn’t shown any action on his part, don’t you just have to say “I can’t fix things alone, so if you’re not going to participate, I’m leaving you” ???  I don’t believe in ultimatums generally, but how else can one put this after spinning your wheels for so long?  And is it OK to say this?  I said it this weekend.  And the answer I got was “but if I do x,y, and z and I still screw up, then what?”  

I can’t fix his cyclical thinking of failure.  Nor do I want him to seek help just because I “nagged” him (which is SO far from true because this has been an issue for 15 yrs!).  I’m just so tired.  Physically and emotionally.  Did I do the right thing?  What do I tell him when he says part of his failure to act is based on his fear that nothing will work??