First things first, once you know that your boyfriend had an ex-girlfriend, it is time for you to talk to him about her. It might not sound like the best idea in the world, but there is no better way to go about things than confronting them. Talk to him about her and ask him the reasons why the two of them broke up. Also ask him about how he feels about her now. Make sure you do not play with his feelings throughout this process. If he does not want to talk about it, leave the topic.
If he is not willing to say much, confront his friends. Take them under confidence and inquire from them about your boyfriend’s ex-girlfriend and how everything was for both of them when they were together. Friends can provide a better insight on the whole thing since they have their own perspective regarding their friend’s relationships.
Always remain confident about your own relationship. It’s quite common to ruin the present relationship by inquiring about your boyfriend’s ex-girlfriend. Simply remember that his past means that it is gone and there is a high possibility that he is a changed man now. You can always investigate his previous relationship(s) but never get obsessed.
At times, if you have to, you can also speak to his ex-girlfriend directly if need be. However, try this situation at any cost. It mostly ends up making the situation even worse for everyone. Always be courteous and respectful whenever the two of you are talking.
If it has become too much of a problem and you think you cannot handle it, it’s best to take counselling. A person who does not have an insight on things and is seeing it as a third party might provide you with a feasible solution.