Have you ever gone so far into a character’s mind, that you start writing just from their point of view and forget that you have to construct a beginning, middle and end, and put them through some degree of misery because that is how a story is constructed? When you are so far on their side that you can’t possibly imagine putting them through pain, even though you know, as the writer and creator of this piece of fiction, that the story necessitates such an action?
I suppose that’s an indication that you’ve gone too far down the rabbit hole, and should perhaps remember that these characters are indeed fiction, something you dreamed up, not actual real people, and that they have to suffer in order for the story to go somewhere.
Then you go about making their lives miserable, and feel, on some level absolutely terrible for making a fictional character suffer, particularly because you know who will be playing said fictional character on a stage, and you know they are going to have to go through this sort of emotional crisis just because you dreamed it up. That’s what I call writer’s guilt.