SPOILERS: Everything I write here is a spoiler.
Before the end act of the movie, there is a psychedelic montage (dream sequence). What it represents or what happens within it is not entirely clear to me. Perhaps Peter Strickland did not intend it to be clear to the audience. I don't know.
I will give my best guess to its meaning, but first, I shall summarize the end from my point of view. This will give my guess context.
As for the actual end of the film, I took it to be that Cynthia (older woman) went back to being topped from the bottom by Evelyn. This is a sad ending for Cynthia because she truly enjoyed the one time in which she actually took control over Evelyn. Of course, I refer to the eating-cake-in-her-comfortable-pajamas scene.
The only thing that could motivate Cynthia to return to her unhappy state with Evelyn would be if Evelyn continued to be unfaithful and have liaisons with that other woman (I forgot her name). Then I can see Cynthia giving in to Evelyn's control and specific fetishes in order to regain the relationship with Evelyn. And we also (maybe) see Evelyn walking out of the house at night in the dream sequence. Perhaps that represents her continued infidelity?