How to seem confident when you are not? please help?

I am not confident at all !! I always want to cry whenever i have to perform or read a speech at school. I use to be in the school choir and my music teacher says she adores me to bits but i dropped out because my confidence suddenly went. I even dropped out of the school play because i felt i couldn't do it even though i got through the addition fine. I suffer from depression and am suicidal at the moment so it doesn't help very much. I don't like going to school to face everyone. How can i seem that i am not bothered and am not scared and very confident eve though i am terrified inside?