Have you given sex-bloopers much thought? Sometimes mixing humor with sex can lighten the mood and ease tensions.
Laugh, joke... make funny faces and noises... reminisce about times that were funny or good. Anything to help ease your mind, take away performance anxiety, and remind both of you that you're together for more reasons than just sex.
There's probably nothing wrong with you... your manhood is just obeying your thoughts and emotions -- all too well.
"The weakest soul, knowing its own weakness, and believing this truth that strength can only be developed by effort and practice, will, thus believing, at once begin to exert itself, and, adding effort to effort, patience to patience, and strength to strength, will never cease to develop, and will at last grow divinely strong."
- James Allen