The Sophomore Slump: the Fear is Real

“Give someone a book and they’ll read for a day. Teach someone to write a book and they’ll experience a lifetime of paralyzing self-doubt.”
Writing my second novel is, apart from going to medical school, the hardest thing I’ve ever done. My first novel was challenging. I had to learn the art of story craft, of how to create characters readers want to stick with for almost three hundred pages, of writing scenes that turned pages. But my first novel came with lower expectations. There was no precedent to live up to. These were brand new characters having new adventures. They carried no baggage.
Book two is different. The characters already exist. They have a history. They behave in certain ways. I find myself saying, “Gethsemane wouldn’t do that” or “O’Reilly would never say such a thing.” The characters have taken on lives of their own and I’m afraid to write more about them because I might mess up, get it wrong. They’ve become canonized  (at least in my head) and woe be to she who violates canon.
Luckily, I have an editor who is patient and holds my hand through my meltdowns. She reassures me I haven’t lost my mind, other writers struggle with the same things, and at the end of the day the characters are mine. I can make them do what I need them to do for the story. I just need to get out of my own way and write.  Has anything you created ever taken on a life of its own?