There are times when I lose motivation to keep going on a knitting project. This is often when the project becomes a bit monotonous, such as the body of a sweater for a tall person, or a repetitive pattern that I need to concentrate on.
I have found that it helps if I can see how much I have done every day. It also helps me set a reasonable goal for how much I can get done in a day. I place a marker on the last row done on the previous day then in the evening I can see how much I have accomplished that day. It gets written down in a knitting diary.