I'm working on my third book of the day. Plus two short stories got some words today.
So I think my solution is, "suck it up and deal." Also, "wait for the book to get demanding." Because they do, and I understand the urge to trouble trouble, but really your mom was right, and trouble will trouble you eventually.
It's easy for me to say this, because I've finished books before. But so has Mer, so she knows she can. I wasn't always in this position. I wrote my second book accidentally, while working on a million projects at once. It attained critical mass. Things happened. They turned out to be a book.
This is not to say I don't fuss about what I'm writing before a project goes critical on me. It's not at all to say that I'm relaxed and never overthink it. But I try to be. I don't think it's a bad thing, trying to be.
(ksumnersmith, the third novel in question is The Mark of the Sea Serpent. To make matters worse, the first was The Winter Wars, a.k.a. Not The Moose Prequel, and the second was Kjartan's volcano prequel to DBM. This isn't the extent of your birthday present, but I Just Hope You're Happy.)