I never feel like I fit in clothing wise no matter what. I've figured out a way to make a lot of pocket money and I continually buy clothes and shoes in the hopes that one day I feel right but I seldom do.
May I ask your body type? You don't have to answer here if you don't want to, but I have daughters of various shapes but similar sizes, might be able to help. I am literally more confident naked than in badly fitting clothes, you are not crazy. Those pictures you see online are tailored and styled, not off the rack. And often photoshopped as well.
What do you like about your body? Like do you like the big boobs, or your height or waist? I always think the big boobs look so good in plunge necklines but sure that's impractical for everyday wear. I will very, very, very strongly urge you to find a tailor, if you are curvy, they can take those too-big shirts and make them body skimming so that you don't feel dumpy. They can take in the waist, but also fit it to your shoulders, so that it drapes well.
Lemmy doesn't have the fashion advice threads like reddit did, I kinda liked those. But if you are not too short, I imagine the nice flowy pants and a closer fit on the top, my sister has the big (purchased) boobs and looks so good in that style. If you are quite petite then straight leg or jogger bottoms that fit a little closer and T- shirt that is high hip length (also works if you are tall but is harder to find the shirts). Again, tailor so the front of the shirt doesn't land higher than the back. I don't like dresses but if your waist is the smallest point dresses will be so flattering, close fitting through the bust and high waist then flow downward.
It does depend on what you personally consider assets though, and cultivating an objectivity so you don't just dismiss every look as not you, not attractive, or not appropriate, you gotta look at yourself as though it's someone else in the mirror, your body twin, a friend with a similar build.
I like that I'm curvy and I do tend to V necks because the tittays are still impressive even at 50 haha. I am on the shorter side so some stuff just makes me look dumpy, like long dresses or high waisted stuff. I just did a big clothes shop and did really well, but I always end up feeling underdressed somehow? And I gravitate towards bright colours and prints when other people seem to be in black leggings and runners.
The bright colors and patterns will be such a glorious old lady style though, when you get old!
I live in Florida & underdressed isn't a thing here. I would bet money that all you really need is a tailor, it's not surprising you would feel dumpy if you have to buy clothes based on your bust size and they are too big everywhere else. Especially if you are curvy petite. There are so many gifted seamstresses working out of houses where I live, it's a thriving cottage industry. There is an ancient old black lady who lives behind us that does my kids' clothes when they are going to quinces and bat mitzvahs, proms, all those fancy ass parties you have to look good for, I can't afford actual evening wear and the seamstresses can turn the thrift store dresses into great looking unique stuff for them. Do not learn to sew. Just find someone good at it.
The FDA will hate me for sharing this with you, but when I feel that way, it’s usually because I’ve gained some weight, or feel bloated, and the only things that seem to help are ivermectin and pyrantel. I don’t know if that’s how you feel in terms of clothes not fitting right, but if so, I hope it helps!