Jenny was molested daily by her father. Her mother was dead. She was already fukked up in the head as a kid, and teenager. She knew she couldn't ever give Forrest true love. She would always be a mental fukk up and he couldn't understand nor take that in his life. She did him a favor. She would have broke his heart repeatedly and drove him to an early grave. She ran hard and fast to escape her past. She loved Forrest. Loved him like a brother. Maybe even loved him as a true to God good man, but she knew she was damaged goods that didn't deserve him. Thats why she left after they had sex. She knew she would be momentarily happy. He would spoil her. She could buy anything. But deep down she knew she needed help before she ruined both their lives. I think she left with the intention of, and got the mental help she needed before she died. It just so happened she also had Forrest Jr. 
