How To Strengthen Penis During Intercourse?

Hi, this is praveen.I am 25 year old male. I am working as a s/w engg in hyderabad. Few days back i had a sex with my GF.I have participated nearly 10 times in last one month but i didn't get succeeded atleast once. My pennis was not getting stronger during sex. For one time i treate as due to nurvous feeling i falied but it happened same for 10times. This problem hitting me really very hard doctor. In normal time also my pennis getting much stronger for few seconds only. Please suggest me prescriptions or food habbits or whatever to get rid of this problem.Kindly resolve this issue ASAP.