I read this today in Juliette Aristides book "Classical Drawing Atelier." It helped
Daniel Parkhurst wrote," Don't undertake the painting of a head without considering well that you are likely to have trouble, and the trouble you will have is most likely to be of a kind that you don't expect. But, having begun,keep your heart and your grit, and do the best you can. Remember that you learn by mistakes, and failures are a part of every man's work, and of every painter's experience, and not only of your own."
M. Bertin talked of Ingres' frustration "Ingres used to weep and I spent my time consoling him."