I had this problem too. Since I wanted to write all of them, I just had to confront myself and say "Look, prick, you will work on this one plot until it is finished, and then you can start the next one. Got it, bub?"
It seemed to work. I wrote a few short stories here and there to take a break from the pressure of the novel. That helped also, when I was stuck in the GSM. Took some stress off to just finish something.
So, I suggest just pick one and do whatever you need to do to make yourself work on that one alone. Even if the motivator is just to finish one of them so you can get to the next one. Whatever makes you stick.
That's just what worked for me, anyway.