I read in a sex column called "ask isadora" about the foreskin that wont go back she says:
"I'm afraid what you have to do is see a urologist. There is a strong possibility that you have phimosis, a condition common to about one percent of men, which basically means the foreskin is too tight, leading to difficult and/or painful erections."
So it seems like you might want to really have a doctor take a look at it... or at least do a search for PHIMOSIS and read up about it.