It was Jenny's fault that her life was bad, because she allowed being abused to affect her. Forrest just moved on from everything - that's how you should think.
What does someone's past history of assault matter in this context?
It gives people an idea on how to do better. Why is it a big deal.
Not everyone experiences the same level of assault and trauma. People may be better at moving on or even hiding their trauma than others. Doesn't make it okay to dismiss a person who is struggling more than someone else and you should be more understanding in those cases.