There has been ambivalence on Lisa's part, but she's still deeply grieving. On reflection of near and recent events she's remembering Jessica's issues with anger, paranoia, tranquility, and even episodes of happiness. She's also generalizing her behavior over long term periods. Many people have described Jessica as "always happy", "very positive", "really friendly". It seems apparent to me, the one thing Jessica wasn't is very happy, her life seemed full of sadness IMO. There is going to be some inconsistency and ambivalence in a grieving parent, where you believe they may have had a troubled child, or a troubled relationship with that child.
I hope that somewhere, the draft copy of that book still exists as rumors I've read suppose. It would be a great insight into the life of a girl who didn't get to live what may have been a wonderful life.