Hi @Sooze86 ,
You’re not doing anything wrong! The problems is that all bras fit differently , one size might fit perfectly in one make of bra but yet that same size might be a disaster in another brand.
Years ago I got fitted in a well know lingerie shop and I was in that fitting room for hours, eventually coming away with a very expensive bra…but at least it was the right fit! I’ve tried all the measuring stuff myself and I’ve never yet managed to get a bra to fit using that method. You’re right, try on as many as you can , its the only way to find the right one. The wires should be flat against your body between the breasts, you should only be able to insert two fingers behind the back strap, always try on the the loosest hook setting and make sure the breast is fully inside the wiring and not bulging over the top. It’s a minefield!
And now, of course with the situation as it is it’s not possible to get a fitting or try anything on before you buy, so god know’s how anyone is supposed to buy new bras if they don’t know their exact size !