I’m terrible at trying on bras, they fit in the store but not after I buy them

I bought some bras, they seemed to fit well in the fitting room, bought them and now that I’m actually wearing them for an extended period of time, they don’t fit. It’s the same problem as always, I get the smallest band size I can manage to hook together, and yet it shifts and slides up and down and painfully chafes. I suspect the cup size is too small, but why did it seem to fit in the store? It only stops fitting after I buy it (and can’t return it).

Of course it’s my fault, I went in looking for a 38K and left with a 38J. The employees at bra shops seem allergic to giving large cup sizes. I ask for a 38K, they bring a 38I. I told them that was too small and they brought the 38J and it seemed to fit so I never bothered to try the actual size the calculator gave me. I‘m so frustrated with myself.

Is there any actual way to tell if the bra fits? If the gore sits flat against my chest, there’s no spillage (after scooping and swooping), the band is as tight as it can be (I always put the bra on backwards so I know the band doesn’t seem too small when it isn’t), and yet it just doesn’t fit. I know I should have tried the bigger size, but how would I have known if I needed a size bigger than that? Just keep trying them bigger until I find one that’s too big?

I’m too afraid to post any pictures of myself on here, but I don’t know what else to do.