What is a mistake anyway? For me it is when your drawing looks differently than what you had planned or expected. You expected your drawing to look more like the object you are drawing, and when it does not, you call this a mistake. However, a fundamental part of the drawing process is being able to make adjustments and changes as the drawing develops. If something you draw does not fit in with your original plan, instead of stopping, try to revise, regroup and readjust your drawing. This is a creative change!
I have found that I get better at being flexible and making adjustments the more I practice drawing. One method you can try is to work on drawing for about half an hour and then stop and walk away. When you return, you will be able to look at the drawing with fresh eyes. Now you can regroup and readjust to make creative changes. I find that my final piece is more successful when I allow myself to shift and change as I am working. Also, remember, your drawing is an artistic expression. It is not a photograph. Often these creative changes can add charm and character to your image.
Try it. Allow yourself to stop and see what you have drawn. Ask yourself if there are any adjustments, revisions, or creative changes that you can make.