Winter Doldrums

This is what happens to me in the winter. Probably for some others too. It doesnt even have to be cold and nasty, although that does make it worse.
In writing terms, I dont have a lot of experience, but I've had some major spells of creative blahhhhh this winter. I know I shouldnt complain, especially since I drafted out my young adult novel in insanity mode for 8 straight days. Of course it helped that I didnt work, and the book was much shorter than my adult books (1/3 shorter, to be exact). Still, making that whole plot and everything in 8 days is definitely a record for me. Makes me wonder if the whole thing is complete rubbish and only I like it. Anyhow, after I wrote it, I was looking around Amazon and found a book that sounded a LOT like my book, although I'd never heard of it before. It made me a little discouraged though--someone else had the same idea first, it seems. So I'm going to have to see if it really is at all similar, or just the concept is sort of alike. It just goes to show that old saying is true: no one has had an original idea since Adam (that's Adam in the garden of Eden, for all those unreligious folks.)
Anyway, that drafting is done and now I'm frozen in indecision. Should I go back and change the first book a lot? Should I just start the sequel and change the first one later if needed? Or should I give up on that story for a while and go edit my fresh-from-the-editor other book? I need to add about 5 or 6 chapters, I think, which might take some time. But now that I've finished that series, I think it might actually be easier than when I wrote the first one. I know the characters better now, so adding some from other POVs shouldnt be as hard as it was back then when they were still not fully developed in my mind.
So much to do...so little inspiration/motivation. Waiting on my slump to end. Bleh.