I don't think there is any cure for this problem except setting the built in alarm - "okay, I'm not allowed to stop playing this one game until this alarm goes off"
usually if i just play a game long enough i can stick with it, its just getting past the boring ancient times that i have trouble with..
You find the ancient times boring? I think ancient -> renaissance is the most fun with the late game being the boring repetitive part. I find myself starting new games just to try new things and stopping most games just prior to the industrial age.