Isn't it ironic:
there's him, thinking "she is such an incredible magical creature, I don't get what she sees in me, she deserves somebody better than I am"
and there's you, thinking "he is a wonderful person, he deserves something better than what I can offer, a monogamous relationship with somebody who will be able to only be with him, I am not good enough for him to adore/stay with since I can't provide that".
What if you both just tried to let the person whose responsibility it is to make sure they are not selling themselves short, i.e. themself, worry about their own needs, and trust the other person to take care of themself? That way you can both truly enjoy what you have as long as it feels good, without worrying about the future time when it may no longer feel satisfying.