I am 21 year old. I have huge saggy boobs. It's giving me a lot of pain. What to do?

You need to schedule a consultation with a board certified plastic surgeon in your area (link below) to be assessed in person. Although there will be some sagging with time after a breast reduction -- no one can fight gravity -- it will not be what you are experiencing now. Best, Dr. Nazarian

It sounds as though you would be an ideal candidate for a breast reduction procedure which would help to alleviate many of the symptons you describe.  Discuss your options with a  fully qualified plastic surgeon.