stop being so jealous

I’m scared to even be writing this, because I don’t know if he’ll read it or not…but here goes:
I trust my boyfriend completely…or at least I think I do. But with my currenet situation and the way I’m feeling, thats pretty hard to say.
I guess, “I want to trust him completely”, is more like it. Anyway, He has an old girlfriend thats he’s still friends with. They were dating for around a year I think and I know they broke up in good terms. I’ve never met her, but I know how he felt about her..and I know they we’re in love. Recently, he told me he was going to go to a concert with her. And immediately I felt my face get hot and my head start to pound.
He asked me if it was okay with me, and I lied and said yes and that it didn’t bother me. But it’s all I have been thinking about. It’s not that I think hes going to do anything to hurt me, but i just can’t see him not developing feelings for her again. I mean they broke up because of distance, so its not like he wasn’t attracted to her anymore, and vise versa. I’m just worried that being with her will make him realize that he misses her or something. And its making me nuts.
I don’t know what to do…if i should tell him how I feel or not. I don’t want him to think I’m possessive. I don’t want to be the kind of girlfriend who tells they’re boyfriend who they can and can’t hang out with. Its not fair, and I’m not like that. But..Any other girl would be fine. But just knowing his past with this particular girl is making me scared. And feel sad. I’m lost I don’t know what to do.
Someone please give me advice. Is it okay for me to be having these feelings or do I need to get over it and let it go? Please, tell me what you think.