I currently have 2 projects I've let fall to the wayside (out of 6 projects) - both have been left behind to make way for first NaNoWriMo, and then the sister-project of this month, Script Frenzy. I finished the NaNo in December (124k) but I won't finish my TV show this month, or the month after.
In fact, in May I'll be going back to the first project I let fall behind, which is a sequel to my first ever novel. Then I'll write the third book in that series. Then I'll consider coming back and completely rewriting the first book (I have my reasons - but I want to give myself plenty of time to mull it over). Then I'll come back to the scripts and finish off season one of the 16-episode TV show, if I can.
While writing the scripts I'll be submitting the trilogy. While finishing off the trilogy, I'll be submitting 2 of my other projects that are already finished. If one of those gets picked up, then I'll put the scripts away and work on a sequel to the one that was picked up.
So basically, that will be the rest of this year - for the most part I'll have one 10k novel that has been left behind till times unknown (next year some time probably) and my scripts which will take so long for so few ks, and not relevant to my career until I've sold the trilogy, which will be left behind to make way for more relevant progress.
But I fully intend to come back to everything.
I think the trick is to stop yourself from writing for just a moment to make an index page of all your stories. In this index write down everything the story means to you. Pick your favourite - don't pick the one that you think will be most successful. Pick your favourite, and write it like the wind. It's like the bandaid trick - fast is painless. You have a wound - you can't finish a project. So you need to be quick - get the first draft of your favourite story all written up in under a month (it's easily possible, even if you have a full time job and a family - you just have to have dedication, or so everyone here says so - personally I don't have a job/wife/kids, but I can still write a novel in a month).
In a month's time, you'll have a finished first draft and a headache and a desire for a holiday. No. Don't do it. Go instead to your second favourite story in that list. Any new ideas you had while writing that first one should've been saved into the file under the heading "New ideas" - don't make notes on what you like about those stories, just write down the premise and move on. Okay, so you go to the second favourite. Now you're wound is starting to heal, so this bandaid can come off slower. Write fast, but allow yourself a little more down time. Aim for a month and a half.
Okay, so you're almost there. Third time's the charm. Pick your third favourite story from that initial list - note that your list of new ideas is growing quickly. That's okay, it's natural. Just bypass them for a while. Now, here's the real trick. Don't set yourself a time limit. Just write as you feel like it, and any new ideas you get, feel free to write down what they mean to you and what you like about them. If you can finish this third book, give yourself a holiday. Switch over to editing the 3 completed books, and submitting them, before you go back to work.
A nice trick: If, during this third project, you feel yourself consumed with desire to move on to something else, don't worry - you're just relapsing. Set yourself a time limit and write like the wind on the project you were already working on. Consider this a mild infection in your wound, and the time limit is the soothing cream, a helping hand to get you through.
After the 3 are finished, after you've edited them (hint, try editing all of them in one month - you've already developed a work ethic by sticking to time limits, so you may as well try and get used to the pace you'd probably have to work at if you ever became a full-time writer) - then you can go on holiday for real by submitting the work and not worrying anymore - it's in someone else's hands now.
You'll see in time whether you've been cured or not.
(Note: the method I mention - tailor it to your situation. Your mileage may vary.)
Whew. Long post. I'm tired now. But at least I stayed on topic for once!