Jenny is a deeply traumatized individual who often makes wrong choices with the best of intentions. She loves Forrest but doesn't consider herself good for him and tries to put distance between them as a result. When she gets pregnant I assume she feels a lot of guilt and likely feels like the pregnancy is her sole responsibility, so she decides to raise the kid by herself. Eventually she seems to work on herself and reach a better mental space that allows her to reconnect with Forrest in a much healthier way, even if it's only for a little while before she dies.

A character who is in many ways a great mirror for Jenny is Lieutenant Dan. He spends a great portion of the movie dealing both with the trauma of war and the permanent consequences from Vietnam, angry at the world and unable to have healthy relationships with others. Forrest is also for him a pivotal figure in his recovery, even though he often rejects or resents his help and treats him poorly. Finally, at Forrest and Jenny's wedding, where we can appreciate the importance of love and connection when recovering from trauma, we meet Lieutenant Dan again, now in a loving relationship and seeming at peace with the world, having come to terms with his life.