How do you not get jealous?

I (F) have been seeing a FWB (M) but after a while they asked if I wanted to date, I said I wasn’t ready at the time and we went on as normal. After he first asked though I started to see him in a different light since I never considered him boyfriend material before. After some time he asked again and I said I was open to the idea. Nothing happened.

He’s seeing some other people and I’m seeing one other person. But now it just makes me really anxious that he’s seeing others when it didn’t before he asked me if I’d consider dating him. Also I’m still a little on the fence about dating him and he knows that because I’m not entirely ready to date. I still want to continue being friends but idk how to stop being so insecure about him seeing others. Part of me wishes he never even mentioned potentially dating because I never would’ve considered it and have the jealousy I have now. How do I stop being so jealous when I’m not entirely open to dating yet?