How do I kindly explain this to my ex?

I broke up with my ex months ago and he’s still trying to get back together with me. I have no hate towards him but he really traumatized me and I’m still so hurt from when we were together. He doesn’t want me bringing up the past anymore and He claims he’s changed so much and stopped the bad habits he had when we were dating and thinks he can treat me the way I deserve now. The thing is I’m still bitter about how he treated me in the past and I don’t know how to tell him I don’t want to be with him ever again or feel the way I felt when dating him. Like I said I don’t hate him, I can see he’s changed and trying to be a better person but I don’t want to risk getting hurt again. Dating him felt like I was dating a big kid when he was actually 6 years older than me. How do I nicely explain this to him?
How do I kindly explain this to my ex?
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