I asked an instructor of mine what he does on his “bad” days of sketching because I was stuck in a rut. He said he just does something else, like 3d modelling. However, some of my best sketches have come right after I say to myself “Man, I’m really sucking right now” and spend a good chunk of time on a random, loose sketch that I’m not really paying attention to which comes out pretty decent. I don’t really have a solution since I try to work through it to get the weeks assignment done, but good luck!