Personally, I know that there are times when I just want to give up on my manuscript or set it aside for a little while. But it’s important–very important–to write something every day. And it can be bad. Believe me, learning to let yourself write badly is an important thing. The first draft is going to be bad. That’s why we edit!
And even if what you write is horrible, you might find one sentence–one idea–that might have something to it. And if that happens, then what you wrote wasn’t a waste.