Wecome to SR and congrats on your decision and 84 days of sobriety. The "brain fog" is very common in early sobriety. Alcohol is a central nervous system depressant, so it literally changes our brain chemistry on a pretty major scale, especially if we drank daily for a long period of time. In essence, your brain is literally waking back up...and it takes some time for it to happen - years of abuse don't just get undone in a few weeks unfortunately.
The good news of course is that it will definitely improve over time. Hope you can stick around SR and join us for some conversation.