The best way to broach a difficult subject is to do so out of the heat of any argument, and in a neutral location. So don’t forget a conversation about this issue when you’re suddenly angry that he’s brought up her name. Instead, wait until a weekend afternoon when the two of you have no other pressures, like work schedules or family obligations, to go have coffee or take a walk in a place that is not your home.
The goal of your broaching this subject is to share your feelings and allow him to respond to them. If you put him in a defensive position, you’re not going to get what you want. Instead you’re going to get a fight. So see if you can keep your tone and your words as neutral as possible so as not to incite his anger.
Tell him that you feel sad and even rejected when he brings up his ex-girlfriend. Don’t blame him. Just share your feelings. Instead of telling him that you get angry, tell him your deeper feelings — that you care for him so much, and you wonder when he brings her up if he’s not missing her and wishing he was with her.
He may not even realize how often he’s doing this, and you’re bringing it to his attention, gently, may help make him aware of a problem he didn’t realize existed. Allow him to tell you his thoughts — and don’t criticize them. Again, avoid backing him into a corner, and keep your feelings so honest you don’t have to be angry — just sad. You’re much more likely to get to the bottom of why he’s bringing her up so often, this way.
I hope that helps. 🙂