Correct breastshield fit?

How do you ladies know if the flange of your breastshield fits correctly? I asked my LC about my fit b/c I have bruised and cracked nipples, and she kind of brushed me off and didn't tell me what size shield I should be using. I've watched videos, but I swear almost every nipple looks the same: the one with too big a flange, the one with a just right flange... Idk how to know which size I need!!! 😩