I'm 21 years old and I can't control my hand to pull my hair since I was 15 years old. I know its call "trichotillomania". I tried to stop but I can't?

See trich.org. Treatment of choice = habit reversal training which involves a) awareness training; b) training in substituting behavior that competes w/hair-pulling; & c) social support. But hrt has not proven sufficient by itself. Treatment tends to be more successful/enduring when combined cognitive behavioral therapy, acceptance & commitment therapy, dialectical behavioral therapy &/or psychopharmacology.
Hair pulling Rx. You are correct in the diagnosis. Sometimes you can't stop on your own. I always recommend a two-fold approach. 1) get therapy. Identify triggers, things that might be making you anxious (where you are likely to pull), etc. Get coaching on breaking addicting patterns. 2) get medication. The best drugs for this are those that boost serotonin: the "ssri's." combining the two txs is potent.