Writing the book turned out to be easier than fitting it onto a single page. Seriously. Three hundred pages, a year and a half of my life, dozens of sleepless nights — and now compress all of it into ten or twelve sentences, in a way that makes an editor or publisher want to keep reading. I sat down to write the synopsis for my first book and realized: this is a whole separate genre, and I didn't have a single skill it required. I know how to tell a story in three hundred pages. I don't know how to tell it in one.
Every attempt turned into either a dry plot summary ('The heroine moves, meets someone, realizes something, changes') or a jumble of all my favorite details at once, because how could I possibly leave THEM out? Spoiler — you can. You have to. A synopsis has no patience for your favorite scenes if they don't move the plot forward. It has no patience for anything, really, except the bare skeleton of the story — and you, the author, are usually in love with the flesh, not the bones.
I rewrote it, I think, about fifteen times. At some point it hit me: if I can't explain my own book in ten lines, maybe I never really built the plot properly in the first place? A synopsis is a pretty honest litmus test for whether your story has a structure, or whether it's just beautifully written chaos.
By the way, if the word 'synopsis' also makes you want to tear your hair out — right now EditRoast.com throws in a synopsis for free with a paid manuscript review. You don't have to write that nightmare of a page from scratch, alone — the service does it for you while it works on the rest of your manuscript, and all that's left for you is to fine-tune it to sound like you. Given how many nerves I burned through on my fifteen attempts, I'd definitely have used that.