Thanks, CaiHong. It's not an excuse, it's not a justification, it's not a rationalization, but if you're asking what the trigger was, it was having to put my cat down.
I really thought I was on the right path until then. I didn't think I was 'cured' or even 'recovered', but maybe just recovering and coping.
But almost as soon as I left the vet's office, I stopped at a bar and had a couple of glasses of wine. The next day I said to myself 'what the hell, you've already slipped', and stopped off at the booze store to get more.