Dear Nasir,
You need to take comfort from the fact that your problem is not the one with the functioning of your hardware! If there is a physical problem with the functioning of the penis or hormones etc. you will not have had erections with your wife also.
This then means that it is a problem in the 'psycho-sexual domain', which is an origin in the mind. There could be several possibilities - guilt, fear,shame, communication problems, secrets, lies, fear of STI's or pregnancy etc. Do introspect and see a counsellor if needed.

Hello,
Thanks for choosing health care magic for posting your query.
The fact that you are able to perform good and satisfactory sex with your wife suggests that there is no anatomical problem. The problem lies in your mind and probably although you are having sex with your girlfriend, somewhere down under you are not comfortable with the idea.
You should see a psychiatrist who can assess you in detail and make out what is causing trouble to you. There are probably no medications that can help you with this condition.
Hope I am able to answer your concerns.
If you have any further query, I would be glad to help you.
In future if you wish to contact me directly, you can use the below mentioned link:
bit.ly/dr-srikanth-reddy

Wish you good health,
Dr. Srikanth Reddy M.D.

Dear, good to see you here.
You definitely do not have any problems with your sexual functioning mechanism, no erectile dysfunction, no premature ejaulation etc. Over and above you are doing very well with your loving wife.
This clears one point that love and affection are basics for sexual happiness.
With your girlfriend, love and affection is minimal or not available.
Over and above you may be guilty.
Keep cool. multiple sexual life can hamper your physical as well as mental life.
Be faithful with your wife and life, enjoy throughout.
Greetings.