I haven’t worn Cosabella recently. But I did several years ago. European sizing takes some getting used to and experimentation. Cosabella and Simone Perele were my favorites then. They’re very well made bras. Def worth getting fitted if you feel uncertain or don't like the support and comfort you get from this order. I didn't find my Cosabella day bra super supportive. But back then I had smaller breasts and didn't mind. So your mileage may vary.
It's also possible I simply needed a much larger cup and smaller band.
The main thing I've learned from the Reddit subthread is to make sure the breasts are fully and deeply seated into the cups. More than you think! To achieve this, one usually has to go up several cup sizes, and down a few band sizes, to get enough lift and encapsulation.
That is one thing that wasn't taught to me, even in professional fitting. Wish they hold told me that. It seems to make a huge difference for many women. I actually ordered a new bra size just to see what happens by getting more cup support. They mostly suggest European brands on that subthread--Cosabella too I think?