I know exactly how you feel since I have the same problem. I start on a game design and I have great idea which I jot down and then I start work on it only to convince myself that it can't be done and then I stop. After I while I convince myself that it can be done and start on it again.
Like you I could have finished a game by now but I was stuck in a vicious cycle.
What I did is I forced myself to focus and set myself a fixed (and reasonable) deadline. Every day I would work on the game design even if I convinced myself that it can't be done even if it is just writing done an idea without fleshing it out into a proper design. That way you are still working on it and still getting to the end.
I wish there was a magic cure for this that I could share with you but there isn't. You just have to force yourself to focus and force yourself to work even if you don't feel like doing it. Above all make sure you work on a game that you yourself would like to play. If you don't want to play it then you can't expect anyone else to play it. I love science fiction so I am working on a science fiction game.
Good luck and keep us posted.