You are not likely to be impotent. It is said that sex is in mind not in organ. So psychological factors are very important for arousal and erection. But there can be physical factors which may need work up to ascertain the cause. If you have erection when you get up in the morning, chance of psychological factors are more. For an healthy individual like you, this should be not worrying. The most important thing is that it a single erection problem plays in the mind. This can give in the back of mind a performance anxiety and you start watching yourself more closely. This close scrutiny of yourself can loose the concentration from the sex and you get a bit softer. Please remember sex in not in organ but in mind. Once you start enjoying it you will regain the same strength. Nothing happens suddenly in sex, it is like sports, sometimes you excel and sometimes you are mediocre. Don't put yourself under the scrutiny and just start enjoying it. Things should come back to normal.
You may need psycho sexual therapy which helps to reduce performance anxiety, develop the new sexual skills and knowledge, modify negative sexual attitudes, and improve communication between parters.
Still if you feel that problem can be organic; you may consider to consult a urologist who would advise you the following investigations;
1) Testosterone levels
2) LH estimation
3) Thyroid function tests
4) duplex ultrasonography
5) Ultrasonography of testes
7) penile nerve test,
8) snap and strain guage test,
9) penile biothesiometry.
Please feel free for your follow up questions.
I would be happy to assist you further, if you need any more information.
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