I know it sounds stupid but I just stopped.
There was a lot that came before that and a lot that came after.
So what came before was cutting down. I cut down to 2 days a week, then 1 because one of the days was before "long run" day and was affecting my performance so it made sense. Then 1 day felt a bit like "why am I bothering? I can see my sleep is worse that day, I feel obviously worse on a Sunday morning..."
What came before that still though was changing workplaces to somewhere where most people didn't drink or drank but not to excess. So socially I started to hang out with people who didn't drink and I started to see myself as a non drinker.
But eventually it came down to one day. I'd got something in the diary 2 weeks later I would have liked to drink on so I considered delaying then I thought:
"There will always be a reason to delay."
And quit. I just decided "I'm not going to drink this week and I'll see how that goes. Maybe it will be forever."
First couple of weeks were the worst but not terrible, perhaps because I'd tapered down (I was easily a one bottle of wine a day person at my worst so going from that to nothing might have been a bit much.)
Ooh this is all in reverse, I do apologise. But I think the memory I cling to is from this time last year. I was away in another country doing some work for a client who is also a friend and has been for years. She'd arranged my hotel and paid for it. After I'd delivered the work, I got some wine to drink in my room (I'd waited till the work was all done.)
That night, I got drunk and ended up falling asleep, wine in hand. Wine went on the pillow etc. I was so embarrassed. I left money for the cleaner, hoping they'd just clean it and not escalate it to my client. (I never heard anything so I'm hoping not.)
That memory is something I cling onto. It's not the worst "rock bottom" moment I had but it was one which fills me with shame. While I didn't give up that day, I did start to reduce my intake and that was step 1 for me. The moment where I realised it wasn't something I had to chill out, it was something I had to get drunk. It wasn't social, it was a drug for me. What is fun about waking up in a cheap hotel in Europe covered in booze? Nothing.
I don't know whether everyone needs a "why" like that but I think it's helpful to have a thought that keeps you from going back if that is what you want.