Everyone has low days. When I do, I just power down and do something else.
I can't imagine letting a low day discourage me. My work is way too important for that.
If you ever think "What's the point," "Is this going anywhere?" "Will anyone care?" or "Should I just give up?", that tells me that you're not sure about how important your project is. Get sure. Is it too important to let a few speed bumps discourage you? Decide. If it's that important, then get back to work. If not, do something else. There's no in between.