The condition you are in is natural result of seeing porno and doing masturbation.I hope you have not injured your nerves by masturbating for 30 minutes at a stretch. You need to take the following steps.
Stop watching porno.
Distract your mind from sexual activity.
You can achieve all the above things by doing abdominal breathing. Google the phrase- abdominal breathing. learn this breathing. practice it for 15 minutes twice a day. After a month do male deer exercise, kegel exercise, anal peripheral prostate massage. Google the above exercise to learn them and you may practice gradually. or you may take advice from a doctor.Read messaes from other members to get more suggestions.