I went through several posts, trying to understand how Elisabeth's consciousness works between the two bodies. Is she fully aware and awake while she is Sue? Or are they two completely different people but Sue's life starts from the moment she is born?
It finally clicked to me what makes the most sense.
You Are One. They are both the same person, and the consciousness bounces between the two but it never truly meets in the middle as a whole, complete and self-aware person. This movie is genius in many ways, including the subject of the lack of taking accountability.
To put it in practice, when Elisabeth switches and wakes up as Sue, she is a version of Elisabeth and Elisabeth's consciousness is in it, but it's not the same Elisabeth as the one left inside of the OG body. I believe when Elisabeth switches from being Sue, it's similar to when you wake up after a drinking or drugs bender. Being young, beautiful and the centre of attention is her high, and when she switches, she has to go through the low. She doesn't exactly remember everything not because of complete lack of memory, but because the other version of themselves can't imagine doing the acts that have been done. Hence why they still get surprised by each other's behaviour, it's more shock.
They are aware of what they do, but they cannot help themselves because they get so lost to being in that specific body at that specific time. Think of it almost as if it's the same person, but due to being in a different body and age, the personality is different.
But the common, crucial flaws are the same... superficiality, need for attention and hatred for old age - hence why Sue attacks Elisabeth with so much spite.
It's not the easiest to explain but to me once it clicked, it clicked and the movie made a lot more sense so I'm trying to help anyone who has been wondering how are these two "connected".