Sound familiar to you? It did to me. And to Ellen, who wrote her an encouraging and relative-like letter.
This is what I wrote.
Welcome to the wonderful world of writing a novel. Seesawing is NORMAL. We all do it, to one extent or another. Once you've written a novel (or two, or three) you learn (or your partner and friends learn) to recognize the symptoms before they become acute, and apply hot tea and soothing noises to avoid an acute flare-up. Or not. We all hate them. They suck--energy, confidence, time. But they happen.
The most telling sentence in your screed is "(well, except for a little bit yesterday which was OK.)" It (where "it" is getting sentences to make sense and say things you want them to which make the story move forward--let's call it "flow") hasn't flown the coop. It's just become subsumed in your uncertainly about What Happens Next. This is a pain, a dark night of the soul, and the moment that ultimately divides Writers Who Will Finish A Novel from Writers Who Won't. You just have to figure out how to get through it.
Ultimately (as you know, because you said so, very clearly), you need to figure out how you do it yourself. But there are things you can try.
1) Don't write a synopsis until you're ready to. And when you're ready, do what Ellen says. [Which was: "Don't give up on your synopsis. Write it as a 2-paragraph overview - as you would a 250-word review of the existing book, glossing over details. Seriously; pretend you're a reviewer who's reading the finished perfect work - or a kid blogging about her favorite book 2 years after it's come out."--excellent advice, which I intend to take myself, as soon as I'm ready to write a synopsis of my current First Draft in Progress]
2) Try writing the Good Parts Version. This is defined as "All the Stuff I know has to happen and I feel like writing, more or less in this order, but we'll see."
3) Bull on through regardless, throwing words at the wall in the hope that some will stick. One member of my writing group, when writing her first draft, writes scenes that seem to happen in Real Time, in which the characters sit around cooking dinner or mending harness while talking about the weather or the crops or their love lives for PAGES AND PAGES, which is fun for us to read, but not ultimately useful to the plot or the structure of the novel, in the course of which she will write [FLOUNDER], which is obviously exactly what she (and her characters) are doing. She doesn't rewrite them until she's finished the draft, at which point they either disappear or get so completely rewritten that maybe only the setting and one line of dialogue survive from the original. She finds writing them immensely useful, though, however seemingly inefficient, for getting to know characters, for creating an atmosphere or details of her world.
4) Talk the next part through with someone. You need someone who will ask you a lot of questions, who will make bad suggestions so that you can contradict them, who is not at all invested in your taking their advice if it doesn't work for you.
Whatever you do (and I'm sure there are other things I've never tried or thought of), get that shitty first draft done. You can't fix something that doesn't exist. You can't rewrite a faulty text that's still mostly in your head. You can't experience the thrill of making a recalcitrant scene or section work by changing a paragraph, cutting a sentence, adding the perfect line of dialogue if you haven't written the clunky version first. Do whatever it takes. That's the prime function of NaNoWrMo, which is designed for those who like to know that they are not alone in their suffering. If you're a page-count junkie, set yourself a daily goal (me, I'd rather type naked on Riverside Drive). If you like timed tasks, make a rule that you have to write for a certain amount of time a day, whether you're enjoying it or not (that's what I do). Sometimes I start writing, "This scene has to have these characters in it, and it would be nice if somebody mentioned the 800 Gorillas in the corner in the course of it, and maybe the Hero could cry, and how on earth am I going to make that happen? Well, he's scared of mice. No, I don't need to know where the mice come from. . . ." Etc, etc, until I'm writing an actual scene, which may or may not bear any relation to the scene I started out describing because some days are like that. Do something else entirely. But keep writing. Please.