I work on only one quilt at a time. I get easily distracted while quilting so perhaps I shouldn't have put my machine next to a window "oh look, a squirrel". I will look at that distraction for minutes at a time.
The way I force myself to finish one project before beginning the next is to have my next project lined up, pattern, fabric, etc. right where I can see it every day. That way I finish what I'm working on so I can begin the fun of the next project.