I think one thing is to drop the word, "slip." It wasn't a slip. A slip is accidental. Drinking is a voluntary event. You wanted to drink, so you did. Your hand, your mouth.
I also think since you live with a drinker it might help you to set some temporary boundaries around his drinking. What that looks like is up to you. Leave the room, leave the house, do something to stop that fear-of-missing-out thing. Make a list of some go-to activities and keep it in your wallet.
If you can look at it as his illness, not as a party, that would probably help.