I found that drawing a lot on scrap paper or cheap printer paper and discarding each drawing, good or bad one after the other helps to detach yourself form the final result and focus on the process of drawing. It's hard to be stressed about a drawing when you know in advanced it will be discarded. It gives you permission to be imperfect. Good for warmups or practice time. Alternatively, instead of throwing away the drawings, sometimes I store them in a folder or a box for comparing the progress later. I bought a Galaxy tablet to save on paper and I just delete my current layer between each drawing. Alternatively sometimes I make the current layer invisible and create a new one for the next drawing. Same principle as above.