Help please its a stressfull thing in my life

Patient: Hi i am a 19 year old male and i have a problem maintaning an erection and basicly getting an erection i can get hard but i feel i can get harder i feel soft and even then i cant stay that way with in seconds of geting and erection i lose it . i feel i cant get a full erection its not the size of my penus that bothers me its that i cant get hard i need help is this common or is this serious ? wat can i do to fix it ? am i to young for viagra ??