Haha, I know how you feel. I don't know if you are actually after any "assistance" with this, but I find the best thing to do is write it out anyway, or atleast a very detailed outline, but continue only posting the fics you are. That way you can write the new fic at your own pace without your beloved readers hanging out for an update that you may not have because of all the other fic/ideas that you are writing...
I dont even know if that makes sense but that's what keeps me sane lol. I have, like, seven story outlines on my laptop but am only writing one