1. Stay calm and don't rush to react: When he mentions his ex girlfriend in front of you, take a deep breath and try to stay calm and composed. Don't immediately show dissatisfaction or jealousy, as it may make him feel that you care too much.
2. Listen and understand his motives: Try to understand the context and motives behind his mention of his ex girlfriend. Perhaps he is just sharing past experiences or seeking some kind of resonance. Listen with an open mind, but without delving into the details.
3. Express your feelings in a timely manner: If his remarks make you feel uncomfortable, you can express your feelings in a calm tone at the appropriate time. For example, one could say, "I understand that your past experiences are important, but when we are together, I prefer that we focus on the present and the future." At the same time, it is important to respect his feelings and avoid accusations or arguments.