What causes a guy to go soft during prolonged period of sex, I am a 20 year old male who is a full time student during first shift and needing employment so I have some stress but when me and my wife have sex I can perform normal and she has orgasm but before I can I get soft. I do feel overheated and sweat more than normal but I don't feel tired. I don't always do this in fact if I do and 15 min later we have sex I can or vice versa. I just don't want to go to a doctor unless its necessary

Try different positions, like maybe her on top doing all the work so you don't have to. Sounds like this is putting pressure on your relationship and maybe you're anticipating it to happen so it does. Just keep a positive outlook and tell yourself you are going to finish and perhaps you will. Many men I have been with have similar problems, but it has gone away with time.