you have to trust that they ARE in the past, and if they are still friends with an ex lover then you have to show them that you support their friendship because you never know how close they are as friends. what ever you do, do not say its me or her/him because you never know who they will actually choose, its a cocky move to make and it is a big risk to take and to be honest, is it really worth it?
sorry to say but you will just have to get past it, try not to think of them as his/her ex lovers but just as their friends, or as people he/she knows...
sorry its so long, thanks for reading, hope i helped you x
Answer
Remember to keep them in their place, example "old" lovers - in the past. You are the present so do not worry about the past. There is nothing you can do to change it anyway and unless you are both very young, most people are going to have a past with people. Let it go and don't worry about it unless the person is still hanging out or talking to them.