Simple Viagra does not work for everybody, and it could be your frame of mind thinking lets get hard and it wont.
Perhaps you now need to start like the rest of us and do your own searching for the right answer, try other ED forums, for a start there is www.pegym.com www.eathclinic.com www.franktalk.org www.webmd.com after that you could look up DHEA but make sure you read all about first, its a hormon replacement, not knowing your age it may help.
Good Luck
Viagra makes the blood flow to erect your penis. However it does not increase your libido(desire). this must be one of the reason why it does not work on you. as such different people have different experiences.I have mentioned few exercise in some other thread. Please go through it and try those exercises. Major component is psychological. you should be relax and calm.Failure of performance is a usual problem. It takes time to adjust. getting familiar with naked bodies of the opposite sex.