Insecure but trying to deal with it

My boyfriend misses living the single life while he is young but only when I’m not around for longer periods of time. He is an attractive guy that turns some heads which I can’t fault him for. He has told me about these feelings and hasn’t acted wrong in any manor. He doesn’t want to break up currently. Aside from that admission he has been the most amazing guy, very loving and supportive. But since he mentioned his fear of missing out whenever I encounter attractive women I think about him and her organically hitting it off and having a heat time. I don’t think it makes me feel jealous and I don’t feel mad. I feel insecure that maybe I’m not enough. It makes me sad wondering if he could be so much happier without me. How do I talk about this without him feeling like he can’t tell me about his emotions without upsetting me? How do I stop putting myself down like this?