I go through this more often than I would like with my vet. He seems to snap, kind of how you are describing, and even if we spent the whole evening talking about our future and maybe getting married and having a real heart to heart conversation, if he gets triggered things fall apart very quickly. However, what may be different for me is that, while I may have to 'talk him down' the next day, he feels awful for losing it. He was single for 14 years before we started dating (15 mo ago), so he's just not practiced in a lot of relationship aspects (like making up after a fight) but he tries really hard. We've had 3 fights last MONTH where one or other of us stormed out. That's a lot. We've had months long stretches with no fights either. And even if we have a bad fight, and I have to come around to patch things up, I know that's what he wants in the end.
He just doesn't know how to start the approach.
I guess the reason I'm sharing this with you is because your story sounds really familiar. That extreme yo-yo between bliss and utter fear - but I trust, from my personal experiences with my boyfriend (veteran with PTSD) that he loves me in the end. And our relationship is so wonderful that it is always worth the fight. And I hope you and your man can work things out, and maybe decide, at the very least, that you love each other no matter what. No. Matter. WHAT
Good luck :)