Sex Performance problems in young male

Patient: I am a 20 year old male and lost my virginity 2 years ago. I have had one partner since then and have had consistent sex with her. Recently I have noticed that I do not get fully erect with her. Even when she dresses up and everything. In addition I only last one session and then my penis shrinks and can not grow for another 4-5 hours and even then is not fully erect. I am a fit person. Why cant I get full erections and why is my performance so fast?