looking for some advice regarding erectile dysfunction.

Hello am looking for some advice regarding erectile dysfunction. I cant get an erection with my girlfriend of 2 years. I am currently being treated by the urology who have prescribed cialis - 20mg and now 5mg tablet per day. The tablets do work to an extent but the erection does often go after a minute of penetration resulting in no climax being reached and much frustration. They have a 1 in 3 success rate,

The urologist suggests that as on occasions the tablets do work, this eliminates the suggestion that anything is wrong with me in a physical sense. So is this something in my head - psychological rather than physical. The more and more we try, the more frustrating it becomes, however Id rather try than not try. I am in my mid 30s and my current girlfriend is my first sexual partner and the only one Ive really loved.

Has anyone had similar experiences and what suggestions would you have for talking my anxieties. I do often worry about it not happening, I am guilty of over analysing the situation, and always feel very tired.

Any thoughts would be most welcome,

Thanks