You are suffering from hormonal changes causing Androgenetic alopecia causing hair fall. It's completely stoppable even regrowth of hair is possible with certain medications without any side effects. Treatment depends on the grade of hair loss. So, it's a must for diagnosing which grade, you are in for an effective treatment. Please  send photos of your head, by direct online consultation for accurate diagnosis and proper treatment.

Hi, at 21years it is too early to start receeding of hairline. This can be because of excess Hairfall and a family tendency to bald. I find that usually this starts as a lifestyle induced Hairfall which then also starts the genetic balding. You need to confirm if you have a family history of balding in men. It is very important that you start sleeping early at 10pm every night and be in direct sunlight every morning between 7 and 8. This will improve your immunity and general health. You need some hair specific medications which will initially stop your Hairfall and then slowly increase the hair growth phase. This will increase the hair density in your scalp.