I can retract my foreskin when flaccid but not when erect. what to do, because it really hurts during any kind of sexual activity?

You should see a urologist to decide if you should have circumcision or be trained how to overcome that.

Your foreskin is a bit tight. Use a cortisone cream to gently pull back foreskin as far back as possible and pull it forward again. Do this several times a day . Gradually it will be become more supple and retractable( 4-6 weeks). The other choice is a circumcision. I think my method will work Good luck.