Totally understandable
I certainly struggled especially as I knew ow (supposedly mutual friend) and even prior to their affair people had commented on how alike we were, that she looked like my (much) younger sister. He literally went for a younger version of me!
Not just looks either we had various other things in common too which is weird, but she's more passive, more forgiving of a mug than I am!
Now nearly 20 years later, they're still together - and she honestly looks older than me! No doubt at least in part to do with putting up with his shit! (Lazy, repeated infidelity, not great with money...)
I seriously had a job convincing someone fairly recently when they'd seen a pic of my ex and her on sm that she's actually over 10 years younger than me.
But yes it's tough at the start, I was still carrying weight gain from having dd, stretch marks and c-section scar from that too, starting to go grey, was probably dressing more for comfort than style with a wee one to run around after
In the first 2 years after the split I dropped a load of weight, went back to uni which stimulated more interest in how I looked too and I changed my hairstyle and started colouring it.
But weirdly I then regained some weight and was in a job where comfy clothes were more practical, it was around this time they were organising their wedding, I got lots of compliments from ex after handovers of dd which I ignored but found... interesting. Then the night before the wedding he outright propositioned me!
Looks are not the only factor in attractiveness, it's part of it but not the whole thing. Also I've found from interactions with others apart from ex that my body with more weight is attractive to many who actually wouldn't have been attracted to my previously much slimmer body.
It's understandable to focus on this aspect but ime it has nothing to do with why people cheat.
They cheat because of their own insecurities, because they think they won't get caught, for the ego stroking, easy sex, novelty value...
I've had some illuminating conversations with my ex on why he cheated, a big factor seems to have been he freaked out about hitting a milestone birthday, hated the idea of getting older and wanted reassurance he could still "pull" a young attractive (to others) woman.
With all that happened after the affair was out in the open, I don't doubt his claims that he never intended to actually leave me for her, she was only meant to be a bit on the side temporarily but my discovering the affair and not being willing to be treated like that and then very shortly after I kicked him out she discovered she was pregnant meant things didn't go how he planned at all! Well you reap what you sow.
They're now married (he dragged his heels over divorce but told her it was me to avoid remarrying) with 5 kids, I have it on good authority they're both miserable, he keeps cheating but feels unable to end things, she feels trapped into staying because she doesn't want him treating their kids like he did/does our dd (avoided paying cm as much as possible, made little to no effort to maintain contact/a relationship with her). He feels sucked dry financially, she feels knackered out doing all child responsibilities and vast majority of household chores.
My life is not going well at the moment due to health issues but in terms of being single/my relationship life since the split and raising dd for the most part I've had a better time of it than they have.
He isn't worth you worrying about what you did "wrong" or if he no longer found you attractive honestly. It's very rarely anything to do with any of that despite what they might say (cheaters script to blame cheated on spouse/partner, revise history of relationship etc)
So please don't fret on that. Concentrate on what makes you feel good and happy and sod him!