hi, thanks for writing.. i have understood your problem... the loss of erection while sex is mostly of psychological origin..there are many causes like anxiety about sex performances and partner s satisfaction.. once there was a problem while sex, these thoughts bothers u every time u sex... then u are distracted from the sexual feeling and erection is lost... apart from the anxiety related to sex, other social issues which makes u anxious and depressed are also interfering in the sexual performance..so u shud identify your anxiety and try to deal with it... there shud not be disharmony between the sexual partners, sex shudnt be at places where the fear of being caught is there, sex shudnt be in hurry..do not compare your physique with those shown in porn movies...they are professionals... when u have dealt with all the issues, u shud spend more time in foreplay before going for penetrative sex..this will increase the duration of erection..and foreplay shud be continued during penetrative sex also... u can undergo sexual counselling and sex therapy for better understanding of the erotic art of love.. in my opinion u shud first tackle your anxiety by exercise, positive thinking, meditation, yoga and relaxation therapy.. then only u shud go for drugs,.. if u have significant anxiety about sex then u shud take tablet clonazepam [klonopin0.25 mg] before sex...the last resort is to go for viagra, which causes prolonged erection... i hope u ll find this advice helpful.. best luck..