I always find this thread comforting - I know I'm not alone!
Our gig on Friday was in a struggling club. Half the crowd were those we'd encouraged to come along (the band gets the takings on the door), but it was still far too empty to create any kind of buzz.
Even though we'd hardly changed our set, we made silly mistakes and even got lost in one song. We thought we'd played really badly. Personally, I wasn't happy with the clarity of the tone on my new (to me) Jag, so swapped to a Corvette for the second half.
On the other hand, all the comments we received were good. I tend to discount those from "normal" people, but a bassist I really admire (and who gigs around 60 times a year), not only complimented me on my playing, but also my tone.
We also had a few people dancing, which was a bit of a surprise. Mind you, they had plenty of room!
I can only assume that how you feel on stage doesn't always translate to how it appears from the audience.