2 Use a mirror
Working for long periods of time on a portrait without a break can be tricky. First, unwittingly, you become tired. In the beginning you make key decisions, but after a while your concentration certainly decreases. Secondly, the portrait resembles more and more itself: namely the image that has been built up in your mind. It is therefore important that your perception remains fresh. Take a little break every so often. A great tool here also is the use of a mirror. Hold the mirror in such a position that you see simultaneously your model (or photo) and your canvas. You will be very surprised what you see: the same image but completely different.
See me in this video using the mirror. 3:38