You mentioned, it's taken a toll on your mothers, emotional health. Does she have a diagnosis, herself?
What I read, of what you've written, is guilt inducing statements. Fear of your abandoning her, a sense of obligation for all she has struggled with, and now your second guessing your own truth in a fog of shamed statements.
Your honesty of your own feelings is met by a world of tears instead of, a bunch of facts supportive of her own truth. Factually stating, back and forth leads to better resolution of past. You don't seem to want to shame her, just say listen, this is how it was for me. Please respect, this is how it felt. Dismissing your reality, does noone any good. Blaming your father, for how you felt, does noone any good. When she blames your father, it implies that you cannot think for yourself and also doesn't acknowledge the pain, that was very real, for you.
And, if that past turmoil was used to blame, a dysfunction in her marriage to your stepdad, then I say, stay the course in therapy.
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