Help, my bf's mom always talks about his ex!

Ive been dating my boyfriend for 2 years now we aren't living together because I want a commitment before I move in, my bf and his ex lived together right away they dated for 7 years they broke up because they grew apart but they also were a big part of eachothers lives. My boyfriend loves me and wants to marry me one day everything is wonderful with him except he wants me to be friends with his mom and I was all for it until she consently talks about his ex how sweet she was and the stuff they would do with eachother and how she always wanted them to marry at first I just let it go but now it's coming to a point where it bothers me like she wants him to be with his ex. I've talked to my bf and he says I have nothing to worry about and if he wanted to be with her they would be together and he loves me. The last time his mom started talking about it I said it bothers me that she talks about her and she said sorry and she would try not to do it we were at his family dinner on Sunday and she called me his exs name. I don't know what to do it's annoying.