The girlfriend is not the problem. No matter what she says or does, Mr. Berger is responsible for his own actions. He has a choice, to react in a violent, immature manner or to just walk away. In each case, he has chosen to display violence, either against her (car) or himself (cutting).
"You don't wake up one morning a bad person. It happens by a thousand tiny surrenders of self-respect to self-interest." Mr. Berger needs to realize he is in the process of making those thousand tiny surrenders. He needs to stop it now, or it will consume him.
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