I have a questions about my foreskin. So when my penis in not erect I can pull my foreskin all the way over the head, all the way until the string cannot let it anymore, when my penis is full erect I cant pull the foreskin more than half length of head, I fell that I can go more without to much pain but the string starts to hurt really bad and I stop and when I'm full erect my foreskin starts to hurt if I'm erect to long.
PS. Never had sex and is the string on a man's penis and equivalent to a woman's hymen ?
No, the string isn't the equivalent of a hymen. It's called a frenulum and it's a normal part of your anatomy. You have them elsewhere in your body too.
You may have a short frenulum, it happens. Depending upon your age it may be something you outgrow. Many men have difficulty retracting their foreskin until around 16 or so. Your penis can grow into your early 20's too.
If it continues see a urologist. They specialize in the male reproductive system. A frenulectomy (removal) or frenuloplasty (alteration) of your frenulum may provide relief. Both are minor office procedures. In some cases, if you have sex, your frenulum could tear and that may also resolve this. It will bleed and can be sore.
You don't need a circumcision and if one is suggested find another provider.