So me and my partner have been together for over 3 years now. For the most part our relationship has been good but it has been rocky for the past few months, but things seemed to of improved a lot over the past month.
But today he told me that he sometimes finds other women attractive, that he sometimes thinks about what if's (eg what if he was in a relationship with them) and that sometimes fantasizes about what said person would look like naked. He told me that that there's no feelings involved that its mostly just curiosity. He also said that these people have only ever been strangers and that the thoughts have only been brief and that he has never fantasized about having sex with any of them.
I've heard that this sort of thinking is completely normal and as long as he's only looking and not doing anything else then it should be nothing to worry about.

But i can't help but feel insecure and a little betrayed by it. I keep overthinking about whether some of the things hes said aren't true like whether hes thought this way about someone who i know or whether its not just been a brief thought and hes has fantasized over them sexually. That hes just lying to me to spare my feelings.
It makes me feel as if i'm not good enough and like he's considering other people as partners and like keeping them as other options.
I just cant seem to stop overthinking about it and its making me feel really distant from him and completely miserable.

I just really don't know what to do and how to deal with this.
I don't want to be overthinking over things that aren't true, thinking the worst and ending up causing arguments between us.

I told him about how i felt about it and he reassured me that i'm the only one for him and that no one else even comes close. But i still feel badly about it and i'm still overthinking that hes just saying things that aren't true to spare my feelings.

How do i stop overthinking about this? or just overthinking about things in general?