Hot take: women aren’t insecure about their bodies because of advertising campaigns. They are insecure about their bodies because of real social consequences they face from the people in their everyday lives.
People treat you better when you look nice. This isn’t morally “right” or “fair”, but it is true. Intuitively or explicitly, women recognize this, and put effort into meeting society’s expectations of them. If all women woke up tomorrow and said “I am completely at peace with how my body looks”, the beauty industry would likely be largely unaffected, because they would quickly be reminded of the second order effects that their negative self image was driving them to achieve in the first place: a good partner; a better job; more, cooler friends.
From your description, it sounds like she sucks. But based on this, I’d say that either your friends also suck, or you suck, or everyone involved sucks.
Good friends don’t just stick with one side of a breakup because they are more attractive. Even if it’s a nasty breakup, good friends will be there for you just as much as they are there for her.
At the same time, I had a friend go through a breakup. I had met his now-ex, and was the reason they met. When they broke up, I and basically all his other friends cut ties, and hung out with his ex. Why? Because before he’d got together with her, I was already considering cutting ties because he was pissing everyone off, constantly being inconsiderate, and refused to take responsibility for his own problems. The only reason he kept all those friends through his relationship was because he was in that relationship - it felt sudden to him, but everyone had been wanting to cut contact with him for about a year before it happened.
Either way, the reason this sucks so much is because you failed to cultivate strong friendships with good people. Imagine another scenario: she breaks up with you, and you have a close circle of friends who support you through the initial period. Due to your status, notoriety, and relationships in the music scene, suddenly lots of women come out of the woodwork who were always interested in you, but knew you were in a relationship. You go out to a gig, and see she’s making out with some douchebag, but you don’t feel so beat up, because you are there with your friends with a new girl on your shoulder.
And ask yourself: what could I have done to make that happen? Your feelings of resentment have less to do with her, and more to do with your needs not being met in this difficult time. So what you should do now should be to find a way to ensure your needs are met in the future. That might mean improving yourself (asking yourself if you do have an alcohol problem), making new friends, picking up a new hobby for a while, reaching out to old friends to ask for help or advice, or yeah, maybe even moving to a new city! But know: until you can see her hanging out with your old “friends” and making out with another guy and feel okay, you still haven’t overcome your issue.