For a healthy individual like you, this should be not worrying. The most important thing is that a single erection problem plays in the mind. This can give in the back of mind performance anxiety and you start watching yourself more closely. This close scrutiny of yourself can lose the concentration from the sex and you get a bit softer. Please remember sex is not in the 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. If your wife/partners are part of all this understanding you would be back to your best. Don't put yourself under the scrutiny and just start enjoying it. Things should come back to normal.
You may need psychosexual therapy which helps to reduce performance anxiety, develop the new sexual skills and knowledge, modify negative sexual attitudes, and improve communication between both of you. Involving the partners in the counseling process and psychosexual therapy helps dispel misperceptions about the problem, decreases stress, enhances intimacy and ability to talk about sex, and increases the chances of a successful outcome.
Still, if you feel that problem can be organic; you may consider consulting a urologist who would advise you the following investigations;
1) All hormones; thyroid, prolactin, testosterone, adrenal functions.
2) Autonomic nervous system testing;
Penile thermal sensory testing
3) Somatic nervous system testing;
Penile glans biothesiometry
Bulbocavernosus reflex latency test
4) Penile magnetic resonance imaging (MRI)
5) Penile angiography
6) Duplex ultrasonography
7) Combined intracavernous injection and stimulation
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