How to be okay with uncomfortable situations in a relationship ?

I’ve been with my boyfriend for almost 5 months now. Yesterday I found out that his ex girlfriend is in their friend’s text group chat. When he told me I was just kinda like “oh okay” and moved on because I’m working really hard on my reactions and creating a safe space so he feels like he can tell me anything no matter what.. but it makes me feel uncomfortable and I don’t know how to handle it😞

They were together for a year and a half. They share a friend group. They lived together. She broke his heart.. and she’s the reason he claims it’s taking him so long to open up to me emotionally (he hasn’t even told me he loves me much less verbally expresses much emotions).

Realistically, it would be bitchy to say “you either have to leave the group chat or you have to kick her out of it” and I would never say it to him.. but what are some other options on how I can handle this situation or help myself feel more comfortable in this situation? I’m definitely the type of person that feels like exs should stay in the past so I don’t know what to do😞