My girlfriend keeps talking about her ex?

right now I'm in a new relationship that I am very happy with. everything is in order and she and I are getting along just fine. but to every good thing, there's always something not so good. I noticed over time she would always talk about this one guy (shes had other exes but its always this one). I know she means well because he is her best friends brother and it would be hard not to talk bout him. but I don't really know how I'm supposed to feel with this, whenever she would say a story bout him or something I have no idea how to respond to it other than "yeah" or "OK." I'm trying to be a "new me" because I ruined my last relationship fussing over the gf's exes and the little things. she knows she's not really supposed to talk about exes with me since I'm the current boyfriend but trying to be nice I said it was OK so I can't blame her for telling me stories. anyway I know there's supposed to be a question in this but I don't really know what to ask. I'm just feeling very confused so if anyone knows how I can comfort myself about this that would b great.
My girlfriend keeps talking about her ex?
Post Opinion