Why is so hard to understand this kind of thing in this kind of mangas... having memories of that past life doesn't make you that person, is something like having a dream where you're that person, her past life isn't her life now, yes, the MC can take bits of that life into this one, but just that, the past life will not overwrite the present one.
Is like you dreaming of your childhood, after waking up you don't become a kid again or have the same "mind" as the you of that time, right? you just remember the dream and keep living you PRESENT life