I'm too afraid to sing, even when no one's around?

After being scorned by forms of singing competitions (musicals, etc.) no less then three times, i just cant bring myself to open my mouth and sing...i just cant let myself in case im horrible. i know i can do it, but i just...cant. i have to sing defying gravity for a musical theater class, and my practicing has been entirely silent so far. how do i just let myself do it? i cant even do it WHEN THE HOUSE IS EMPTY. i know i can do it, it's just...