Thank you for all your responses! It seems like a lot of people don't mind it when it's someone close to them. I never minded when my fiancé finished my sentences but he doesn't anymore. I told him that I have a sort of... pride problem? When people I don't know well 'help' me out when I don't ask for it, I feel defeated. He completely understood, and even though I didn't mind it with him, he stopped doing it. Even if talking is (really) hard sometimes, I like being able to muddle through on my own!