Jenny was molested as a child by her dad, and though she loved forrest, when she slept with him, she felt like she molested him because of his condition. This caused her to run away and fall back into self loathing and drugs, thinking she just became her own father. She loved Forrest more than anything, that’s why she stayed away, because she was a ticking time bomb and didn’t want to hurt him. When she found out she was dying she accepted her love for Forrest and realized he was the only good person in her life, and the only one she trusted to raise their son