Fellow-sufferers – I feel your pain!   Been there, done that, and got the scars to prove it.

Playing in a bar, to an everyday crowd of people who didn't come there specially to hear you, is always hard work. Performers who enjoy a challenge, who know about "working the room", and who have extensive repertoires to draw on can still make magic happen under these circumstances. But with some audiences, even the ablest and most committed performers eventually have to give up and say "the magic's not working tonight".

When that happens you should ask yourself questions like:
How much am I being paid for this?
How badly do I need that money?
Am I enjoying myself?
Am I learning anything useful here?

If the answers are "not enough", "not that badly", "no" and "no", then perhaps you should look for a different wall to bang your head against.

But if, despite the disappointments, you still feel this is a worthwhile activity, then keep trying. Many people who now do this sort of thing fairly successfully have experienced a few disasters in their time. Billy Joel is an obvious example - and as somebody has already pointed out, he survived, and made a great song ("Piano Man")out of the experience.   

Wassail!