BF is upset because of my trust issues?

We had a huge fight because I saw that someone messaged him on Tinder late last night. He claims he hasn’t used tinder in forever but he still gets notifications and messages from people from time. The thing is, I do have a little trust issues but that’s not my big concern. My concern is that we’ve been seeing each other for 8 months now and we have never talked about exclusivity but we call each other gf/bf. I brought it up that it bothered me because I need clarity.

we both agreed we would only date if it’s going somewhere serious but I still feel like that’s not clarification although he did call me his GF once.

Anyways we’re not really talking bc he says no matter what he does I still wouldn’t trust him, which isn’t true. What’s the best way to resolve this?

W