sing without being embarassed

sing “happy birthday” when 50 other people are singing it at the same time. I have just recently been practicing singing as loud as I can when I am alone in my car, but it takes me a long time to be able to just let loose and do it. Even totally alone where no one else can hear me, I can’t make my voice be heard. I have become fairly comfortable with public speaking, but singing is very hard for me.