Stuck on this painful thought

Earlier in the year my husband was unfaithful to our marriage. It generated an enormous amount of pain in our marriage and we nearly broke up, but we have both committed to the marriage and are working through it. So far all of the thought modelling has been enormously helpful. I am grateful for it.

However there is one thought I can’t seem to shake — I will feel like I made peace around it, but it rises up again, unbidden, in quiet moments or unexpectedly. The thought that: “She is more beautiful than me.” (I have seen photos of her from stalking her instagram).

Though he has told me this is not true, I don’t believe. I have tried to make peace with “well, what is that IS true? Perhaps she is more beautiful than you, but that’s okay.” but that approach makes me feel incredibly sad and depressed and unworthy.

I don’t know what to do. But every time I think that thought I sink into a black hole and it feels like I can’t get out.