Jealous over nothing

So me and my boyfriend are on bad terms right now because when we first started flirting he was messing around w this girl younger than us. Now I know I can’t get mad but I asked him if they kissed and at first he lied but then the told me they kissed. They would meet up 6tH period for a week straight and he would walk her after school for a couple minutes AFTER WALKING ME. Keep in mind he was a big flirt before me and him dated. So when I found out about the girl I got mad and he dropped her for me. Now I’m mad because why did he have to lie saying he never kissed her? Also he would judge little stuff like he thought I would look different under my makeup but that girl also wore makeup and he never thought about that. I know it’s in the past but it gets me soooo angry thinking about them meet up and kiss and now I’m wondering if he had feelings for her? Any advice?

* side note he’s a super sweet guy that always tells me how much he loves me. He writes me cute mini paragraphs every night. Texts me 24/7 letting me know where he is and if he can’t text he’ll tell me why. He’s been all about me since but I need help moving on from getting jealous over his past