For me it just "clicked" one day. I was drinking with my SO and talking about drinking too much and reached for another bottle of wine. He offhandedly said "do you really need more?"
I thought about it all night. Why did I need more? I was already tipsy. Had he not said something I would have drank until the bottle was gone or I was hammered.
I got drunk the next night but I was fully aware that I was an alcoholic. It took the fun out of it. I swallowed down a margarita and it was the very last drink I ever had. It took another week for me to admit out loud that I was an alcoholic.
I guess it was a "moment of clarity" that is described by people. I am aware of the problem now and decided to open the door to sobriety while I was thinking about it. I'm glad I did.