I didn’t find out my grandfather was my mother’s stepdad until I was 12. I didn’t resent, I just thought “WOW!!! This guy who I adore and who obviously adores me doesn’t have to!?!?!?!”
Biology my a&&.
It is not the normal course of affairs for a parent to abandon its offspring. For an individual to be distraught upon learning this happened to them personally strikes me as normal. She’ll get over it. Obviously you’re strong, so go ahead and kick her while she’s down if it makes you feel better.