OP, the same thing happened to me when I was 21. Best thing to do, before it starts really badly stressing you out, is to shave your head. Don't become a slave to your thinning hair, take control and do something that will, with luck, look good on you. I received far more compliments about my haircut within the first two years of shaving my head than I ever received about my hair in the twenty-one years leading up to it.

A guy I knew with a full head of hair came up to me and said, "You know, I always wanted to try that, but I never had the balls to do it." I actually wound up with a sense of pride over it, which was fantastic. What self esteem I'd started losing from having thinning hair (and even before then, my hair was always uselessly wavy and insisted on being unkempt) I'd more than made up for, and now, even if I could have my hair back as thick and full as it used to be overnight, I would still refuse it and stick with shaving.