Sometimes when I lay in bed at night I just think about things that have happened & start to feel deppressed.?

I am mostly a happy person. I have a lot of friends at school. But its just when I think of everything that has happened, & what people have said to me. & what will happen I just break down. I will never think of commiting suicide. But I just can't help it. I'm so happy & joyful during school. I always have a psoitive attitude & try my hardest. I sometimes feel down about myself but people always tell me how pretty I am. I just don't know whats going on anymore? We have a teacher at pur school that if we dont have something he asks if we want to be worthless. I didn't know the answer to a question one day so he asked me if I wanted to be worthless. It really hurt my feelings & lowered my self esteem because he was basically calling me a worthless person who would never get anywhere in life. He tells us he doesnt want to pay for us to get a check from the goverment evry month. I plan on being a very successful person in life & I think about it all of the time. I haven't stopped thinking about that I can here him saying it now"do you want to be worthless?" The more & more I think about it the worse I feel about thinking I'm worthless. Does anybody have any advice to make me feel better