Like stimulation directly by your gf? Watching a movie together with your gf? Do it when you are really stimulated.
Is there any help when you tried some medicine?
Have you tried sx with other g? Probably you can recover the confidence if you have a successful sx there.
The environment is also important. I would suggest you to try some different places to see if that is help.
You can also read some related books before you go to see a doctor.