A person's upbringing and formative years have a tremendous impact on their emotional and intellectual maturity in later years. However, while it is healthy to examine the past, it is possible to dwell on it and use it as an excuse to avoid taking responsibility. If your partner is unable to move on from an unsatisfactory childhood and places the blame for bad behavior on parents, it is time to take a compassionate, yet firm stance in favor of personal responsibility.
Empathize with the feelings he is expressing when he tells you the stories about his past. Discussing the feelings and naming them will help him to face and process the emotions connected to his childhood, but it will also help him to move away from the victimization narrative where he is stuck. Understand that having had a problematic childhood, he hasn't had the help he needs in coming to understand his emotions and in having someone willing to help him bear them. Assist him in this manner to help him learn to be better at facing what he feels.
Discuss with her the ways in which the emotional deficits of her childhood have kept her from learning important lessons about maturity. Show compassion and empathize, but warm her up to the idea that while her problems may not be her fault, they are still her responsibility to repair and that she does have the power to fix them. Explain to her that you understand this will be a process and will take time, but everyone has to go through it whether in childhood or adulthood.
Seeing All Sides
Encourage him to speculate about what his parents' childhoods might have been like. Don't push for him to be sympathetic to them, yet; just ask him out of curiosity. Ask him if he thinks his parents experienced the same kinds of problems in their own upbringing the he experienced from him. Remind him that repeating patterns and cycles of abuse continue until someone takes action to break those cycles. Using a neutral, non-judgmental tone, point out ways in which his behavior is mimicking the problematic parental behaviors that have led to his problems.
Once you have established a strong foundation of empathy and understanding, start being firm with her about what she needs to do moving forward. State that while you want to understand her childhood and difficulties, it is her responsibility to do the work necessary to be able to function with maturity in an adult relationship. Tell her that you will help her, but that it is not your job to be her parent, nor can you be expected to tolerate her shortcomings because of the excuse of her upbringing.