Help with my emotions?

me and my ex broke up 2 weeks ago and he talks about this new girl kim who hes not dating yet but will be. She slept at his house yesterday and i got really jealous and confronted him about ti cuz he said him and kim never hang out alone but they slept together the other night. He texted me today we need to talk and i knows its about me freaking out about stupid things that may or may not be true. How do i prove to him i can change and not freak out about things or that we can be friends with out problems. He wants to stay friends but i think he may reconsider his decision but i really wanna stay friends with him cuz his family is great