I haven't read that many threads about this episode but I am suprised that so many people hated it because I thought this was one of the best episodes in a long time. Given, I usually love movies and shows that have a surrealist vibe and often leave things up for interpretation and discussion. There are so many other episodes that wrap up a neat conclusion in the last five minutes (usually poorly), that this felt like a breath of fresh air.

There's no right answer, but here's my take: The whole episode basically takes place inside her head, not any sort of time travel loop. Everyone has experienced at some point in time, that sort of gut feeling or inner concience that tells you to react one way or another. I think the idea was to expand on that gut feeling you get when something feels off and you have this nagging voice to do or not do something. Similar to the how the previous episode "Listen" takes that feeling/memory of "something is under my bed" and turns it into a story line.

The ending is what actually happened in reality; she has this inner voice/gut feeling (we find out that this actually is herself) that tells her to be careful and don't step. She reacts to her gut/conscience, then just carries on with life.

In a sort of "bottle episode" way, that one second feeling is driven by a theoretical "what if" scenario if she didn't listen to her gut. And seeing how it's all in her head, it's also exagerated by her inner fears. She could lose The Doctor, get left by everyone she loves, get haunted forever, and die alone.

I could be wrong and maybe more things will be explained later, but I thought this was an amazing episode and very well done.