I (20F) am scared of my boyfriend (23M) cheating, how to get over it?

I’m in my very first relationship ever and we have been dating for 3 months now. Everything is going amazingly, he is extremely caring towards me.

I have expressed this fear to my friends and they call it ridiculous because from the outside he seems obsessed over me and I can feel that too, but he has had quite a bit of experience in dating and confessed to me that he even cheated once when he was 16.

I know that he was young and made a mistake but there is still a weird linger in the back of my mind that he might do it again once he gets bored of me and the obsession wears off. He has given me literally zero reasons to think that but I’m still so afraid.

I’m also quite insecure about the fact that he is my first but he has been with a lot of other people before and is far more experienced. Which on one hand is good, because he has learned to be as amazing as he is through his past but I also fear that he might think I’ve been single just because nobody wanted me or something and it might turn him off too.

If other people were so replaceable to him then I could be too. He says that I’m perfect for him and so on but I have no idea if he has told that to others as well. He has been very open and honest with me about a lot of things but I still feel so insecure and scared.