There is a string like rather hard object just under the skin of my penis, and it is hindering me from having sex or anything that involves my penis too much movement. It is located under the head it hurts when I'm erect. I'm only 20 years old.

Greetings,

I suggest you go to a urologist to have that hard object examined.  Your condition does not completely sound like Peyronie's disease and you should receive a definite diagnosis to determine your problem.  TRH