Why can't I find a right fitting bra I think im a d cup when I look at my boobs but when I buy a d cup bra my boobs come from the size?

I dont think my boobs are double d because they dont look that big but these sizes here are not right and often leave me with really bad back pain

is there some special type where the d cup fits more or maybe the form is something is not right

Why can't I find a right fitting bra I think im a d cup when I look at my boobs but when I buy a d cup bra my boobs come from the size?
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