This is my take. Jenny had low self esteem from her fatherly abuse. She was trying to find herself and was more than likely into drugs and booze to kill the memories of her childhood. She could never kill the memory of Forrest though and always came back to him. The movie was about Forrest and his simple minded views of life and his true and unconditional love for Jenny. Jenny was the only woman Forrest had been with and I believe it was his kid. Even if she had said that the child was by another father, Forrest would have raised it as his own simply because he loved Jenny. If you hated Jenny for the way she treated Forrest, then she did her acting job very well, but then everyone hated the way she did Forrest.
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