I cant advocate for myself and it's so frustrating

I can't speak up even when it affects me, I believe no one will listen to me even when I know thats an incorrect fact. I tried to get a coffee and I couldn't even ask for them to correct my order when I know they got it wrong. It hurts. I dont want to be rude. I know its not rudeness to just ask for things to be correct for me but it feels that way. it freezes me. how do you guys do it? I try to breathe and have what i want to say lined up before hand but it doesn't always work.