How to deal with an insecure boyfriend?

He and I have been together for a year today. He's very insecure about me cheating on him because of past girlfriends, almost all of them ended the relationship by cheating on him. I try to tell him everyday he's the only one for me and that I do one day want to marry him. But in the beginning of the relationship before we were official or serious I had this guy who was like a sugar daddy and one time he messaged me while I was with my now boyfriend and he freaked out. Me and this other guy never did anything he just bought me stuff and we never even met in person but my boyfriend didn't believe me and held it against me for forever. Now he always thinks I'm going to cheat on him but he says he trusts me. I'm not really sure what to do, I feel like this will just take time to heal so I shouldn't worry but another part of me wants to fix it now. What should I do? Please don't say leave him because I won't.
How to deal with an insecure boyfriend?
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