I'm missing something here. If I'm interpreting things correctly, the grey-hair Yang/Gang doctor hurt her for an experiment? Or something? And then Dr. Jeong's flashback...I don't understand it.
Something mist have happen.. Physical Stimuli ? Either they went outside or he hurt her physically and it had been so bad/too much that Byeol erased it from her memory.. That's why she no more remember white haired guy.