I lose my erection during penetrative sex

I love and fancy my girlfriend, but I can't keep erect when having sex. I am 42 and otherwise a picture of good health I am 42 years old and always seem to quickly lose my erection during sex. I love and fancy my girlfriend, but this problem has been with me pretty much all of my life. I have been to my GP, had all the tests for pretty much everything and I am the picture of health. I can get an erection but, as soon as I penetrate her, it goes ... Your mind may be playing tricks with you. Such ...