It is a problem of incorrect conceptualization and as such should be treated via doing thinking in your mind.

You need to see and accept the following basic truths:

1) You might be beautiful, but you are not the only beautiful woman on Earth.

2) People cannot be ranked, in general. Some people have green eyes and some brown. Some tall some short. Some bigger some smaller. In the absence of the ability to come up with an ordinal ranking or at least something along those lines, making comparisons between individuals is useless and should not be done. Also, each human is unique and should be appreciated for her uniqueness, and not scored according to some parameters.

3) Your partner is with you for a number of reasons including but not limited to your attractiveness.

4) Your partner is an adult with full decision making capacity and is with you on his free will. It follows that he must find you amazing enough.

I hope you can internalize those, and when you have internalized them fully, your problem will go away.

Some corollaries from the above:

1) When and if he stops finding you amazing enough, you will deal with that situation as you then see fit. Trying to prepare for that situation is futile.

2) Your partner's capacity for esthetic and sexual attraction is a good thing and you would not be with him if he did not have this capacity. His exercise of the capacity to be attracted is a good thing, in and of itself. So if he finds anybody else attractive, it is not a bad thing - it means that he a is alive, which is not bad. So he should not be saying that he "admits" that because you admit to doing something reprehensible and finding another human being attractive is not reprehensible.

3) If you stop ranking people, you will stop deeming others more attractive than yourself.

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The problem is common but is fully due to those mis-conceptualizations. Low esteem is not a great thing to have, but if you conceptualize correctly, it should help even in the presence of a low self-esteem.