I really like the movie, and if you can understand the ending, then you can really appreciate it. People keep ripping on it without knowing what happens, so I am here to explain my theory, I think that Miss Jessel, who was the original nanny at Bly Manor, was Kate's mom (the artist in the pool). She ran away after suffering abuse from the groundskeeper, and that left her with mental issues. To cope with her mental issues, she both painted, and also created Kate (who is just in her mind) to fulfill her greatest regret, not bring the children out with her. In the movie, Kate takes a job at Bly Manor to take care of the kids. She does state that Bly Manor looks familiar. This is probably because she used to be the nanny there. Everything was going well (at least for a horror movie), when Kate starts to see the ghost of Quint (the groundskeeper) who is probably haunting her because he recognized that she was Miss Jessel. Kate then gets a package from her mother of a painting. That painting turned out to be a way of escaping from the mansion with the kids. Kate followed the instructions, and escaped,only for it to be all a vision in Miss Jessel's messed up mind. There turned out to never be a way to escape, and it was all just Miss Jessel's guilt for not being able to save the kids as well as mental trauma.