If this were the pwBPD in my life she would have been embarrassed that she did not appear to all the world as the perfect mom.
My guess is that your exdil was projecting her feelings about herself on to you because she can't manage her feelings. All of her criticism is about
her not you. Don't take it personally (I know not always easy)
Thank goodness you were there to help your granddaughter!
Keep your focus on your granddaughter it's important to have healthy people in her life.
Panda39