How to deal with intense spikes of jealousy and being a chronic overthinker

So I've been with my boyfriend for about 2 months now after taking a break when I broke up with my ex.  I broke up with him because in the 3 months before I decided to break it off, I started realizing that he doesn't really care the same way I did for him.  My ex treated me like another one of his friends and that really put me off.  I felt like I was putting in all the effort in the relationship and he was kinda just coasting along.  Now with my new boyfriend, I've started noticing habits or issues that I never had in my previous relationship.  First of all, I'm starting to overthink everything.  For example: I know he's a slow replier, yet I still get nervous whenever he takes long to reply.  I start thinking stuff like "is he mad at me" or "did I do something wrong", when in reality he's just busy or forgot to reply.  (Context: we are classmates and haven't gone public with our relationship.)  Or maybe when I see one of his friends being "too friendly" or touchy with him I start getting jealous and angry at that friend, even start seeing them as threats.  Then when I step back and think "dude he can have friends, it wouldn't make sense if he got mad when you hang out with your friends now would it", I realize how irrational my feelings were.  Logically speaking, I know our relationship is in a good place.  But in my head, it feels like extreme highs and lows.  With every intimate and loving moment that makes me feel close with him, there's always a moment that just completely brings me down, whether for good reason or not.

Please help me guys