There can be many reasons for your problem..
* medical conditons like prostate infections, low testosterone levels, diabetes
* Use of recreational drugs like cocaine, prescription meds like ZOLOFT,PROZAC
* mental stress, boredom of sex,
You need to get a medical evaluation done by a urologist for further treatment options, and maybe a referral to a psychologist / sex therapist as indicated..
There are many medicines that may help including Viagra, alprostadil (MUSER), L arginine and even sexual experimentation
You can read more
herePlease do not worry you are likely to get better after a medical evaluation,
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Regards
Dr. Gupta