I'm only 13. How can I restore my sensitivity?

You most likely are doing it to often.
Are you using any kind of lubrication ???
Since you are embarrassed I doubt you would and
your age most likely not allowed to purchase lube you
might be wise to use your spit when playing.
Do not use anything other than your spit, do not
use soap of shampoo or oil.

A loss of sensitivity after ejaculation is normal, that is the way a man is programmed.Learn to live with it.It will revive by itself pretty soon and as a young man sooner than others.

Are you circumcised? Circumcised men are generally less sensitive and experience less pleasure. When you are older you can look into foreskin restoration or visit norm.org. Foreskin restoration is long tedious and at times painful process. You cannot regain everything you lost when your parents circumcised you. However, the foreskin like covering you will develop will help keep your glans moist because of the coverage and thus reverse some of the keratinization process that occurred.

My first thought was because of circumcision.