Insecure boyfriend

Hi everyone. I'm starting to feel pretty uncomfortable with how close my girlfriend is getting with a dude she used to have a 'thing' with and I don't really know how to best approach the situation. I have tried speaking to her about it, but it's like she never takes the discussion seriously. If anything, she tries to defend him/their renewed friendship. I do trust my girlfriend, but don't trust the dude in the slightest. It's starting to make me pretty jealous and insecure about myself and I am worried it's going to turn me into a controlling boyfriend. Any tips/help would be highly appreciated!!