[Fit Help] I'm getting super frustrated because I don't know my own size still.

I'm pretty new to r/ABraThatFits so here we go.

I'm [24F] and I still don't know my proper bra size. Like most, I'm really tired of spending money on bras that don't fit.

I bought some 34B push up bras whenever I lift my boobs into place they fit nicely. BUT when I walk around for a bit they'll fall out a bit. I'm not sure if the cup is too big or the bad is too wide. There are three hooks for the bra and I've even tried using the closest one and my boobs still fall out, the band is so tight it feels like it's squeezing my rib cage. I don't know how tight a bra should feel at all. The straps are on pretty snug as well, when I lift my boobs into place and lean forward there's no extra space between my breasts and the cup itself, but after walking around a bit they begin to fall out again.

If it helps, I used a measuring tape and measured around the band area and got 27 inches from it. I don't know if stores sell bras that are bigger than what you measure yourself to be (?).

Sorry that I'm ranting and the poor use of terminology.. I'm just getting pretty frustrated always having to lift my breasts every so often.

Any suggestions/advice?

Please and thank you!

EDIT: Looks like I'm a 28C and not a 34B. Wondering which stores would actually sell my size? I took a skim around Victoria Secret/Aerie and they don't have my sizes.