Boyfriend talks to a girl from his past

So backstory..my boyfriend and I have been together for about a year and a half. The first bit was rough because he was really guarded. He eventually opened up to me. Before we started dating, he was best friends with a girl for about 6 years, he eventually wanted more and she didn’t, so they stopped being friends. He also told me he slept with her once sometime in that 6 years. She is the reason he was guarded.

Fast forward to today, it’s been about 3 years since they stopped being friends. He ran into her at the gym. They talked and said they should grab food and catch up sometime. Of course, I am really insecure and it makes me kind of uncomfortable. My bf says there is nothing to be scared of and he just misses her as a friend because she was his best friend for so long. The last time I went to see him I grabbed his phone and saw that they’ve been texting as well.

I want so badly to just be okay with this because I don’t want my bf to think I don’t trust him. But I just don’t get it. What is the point of it? They stopped talking for a reason.. am I being insecure?? I need opinions. It’s driving me nuts.