If I'm in a relationship with someone how would I express to them that I still want to be friends with my ex?

Okay so basically when I was 14 I dated this guy and we broke up after like three weeks. We were never sexually intimate or anything so not too much happened lmao. He was my first boyfriend and he made me feel really bad about myself. I'm 16 now and I'm in a relationship with a really nice guy who I really like. And I met up with my ex not too long ago. He apologised for how he treated me and he's really changed and he also goes to my school. I told my new boyfriend about me meeting up with him and he was a bit upset but he's fine now. I get that he's insecure but I don't know how I would express to him that I just want to be friends with him and its okay. How do I express that I want to be friends with him?<br><br>Sorry for the long post and thanks for the advice