Same here, it's sometimes difficult to declare a work finished at times. I used to get around this by working on two paintings at the same time usually starting the second halfway through the first. I do this because I don't enjoy the under painting stage but I openly acknowledge it's importance.
yeah most the time when I paint I have two oils going also.. sometimes you just need to take a breath away from one piece and think about another!
The other reason is oil paint can take up to a fortnight between layers depending on the surface and the pigment used.