How do I remain confident?

How do I remain confident? Hello, I'm a high-schooler and stutter. It's annoying when I speech block even tho I don't get it often. My stutter is purely psychological. When Im alone I talk fluently and with my parents I really rarely block. In school I get anxious quickly and it bothers the hell outta me because when I'm alone I can say the word easily. It's because I overthink it, when someone asks me my name randomly I answer quickly and fluently but when I get time to prepare I struggle with it. How do I not overthink it and remain confident?