I am 20 years old and i've had many defeating attempts at sex. the only successful attempts were when i tried viagra (sildenafil). are there other answers or will i have to take viagra (sildenafil) for the rest of my life

At : At 20 years old it seems unusual that you would have erectile dysfunction. Problems like this might be related to hormonal imbalances (testosterone), reaction to medications (especially antihypertensive medications, antidepressants), reaction to drugs( marijuana, etc), alcohol, and stress (performance anxiety or general stress/depression). It seems reasonable to see your physician for further evauation or referral to a urologist.