Could it be something from your past that is weighing you down. Maybe something you haven't worked through properly?
If not then have you tried talking to your girlfriend or a friend about
how your feeling, sometimes it helps to talk to someone and that can help you with the intimacy issues aswell.
also maybe you could try something to help you relax such as yoga or some sort of meditation?
"The Optimist sees the rose and not its thorns; the pessimist stares at the thorns, oblivious of the rose."
"Life isn't about waiting for the storm to pass, its about learning to dance in the rain"