I have those, too. There are days when nothing I paint seems to come out right. It feels like every action taken is just the wrong action. When I'm in that state, I don't like to stop drawing, altogether. It feels like stopping will create the habit, "when things get difficult, quit," which I don't endorse in myself.
So I'll doodle, switch to a different project/commission/style, and sometimes will even go out & sit in a park & gesture draw.
Other times I'll open the painting in a new file, and play around with it until I get somewhere. Having it in a new file often takes the pressure off -- if I mess up too badly, there's still the old file sitting on my disk.