Jim Wendler has a great blog post on this subject called Bad Training Days Ahead.
What do you do when it's a bit rough, when your all isn't all that much, and you just can't match the best you've done before?
Go in and give it what you've got, but cut it a bit short. Save a little for next time. Get your work in but don't kill yourself trying to get something that just isn't there.
Next time, if it's bad again, do that same thing again. When it's good, that's when you push things and drive for PRs and get everything done at maximum intensity.
But don't miss the session. Just get done what you're able to do.
And keep reading Jim's excellent blog.