Why won't my penis stay hard? :(?

I am an attractive, confident 18 year old guy who has had about 20 sexual partners, I've probably had sex over 5,000 times with no error. But recently I've had a problem. My penis will get hard before sex, I can climax from oral sex or hand relief. But when I get it I'n it won't stay hard:( it goes goes completely flaccid. I never have used a condom since my first sexual partner, and have had my share of more extreme sex. Is cialis or Viagra the answer for me?