It seems that you suffer greatly from a deficit of sexual confidence. This normally happens to men who suffer from premature ejaculation. The previous posters are right, you're anxiety is absolutely killing you. A pretty interesting thing happens in the brain when a man feels like he's threatened when he's about to get it on. Men have an evolutionary trigger to do 1 of 2 things whenever they perceive they are in danger while being intimate.
When intimate if a man thinks he is in danger he will do 1 of 2 things either:
1) ejaculate very quickly, so he can be better prepared to deal with the threat.
or
2) Lose his erection very quickly, so he can be better prepared to deal with the threat.
Now here's the kicker. Your brain does not differentiate between physical threats (like a bear coming into your cave) and psychological threats (fearing that you will lose your girlfriend due to bad sex). Both will trigger one of the two responses. So it is very likely that you fear and anxiety of not obtaining an erection or not performing well in bed is ACTUALLY CAUSING YOU TO LOSE YOUR ERECTION AND BE LOUSY IN BED! I actually have an entire blog post dedicated to this (although mainly discussing problem with PE and penal fears). You can find that by clicking
here. Check out that blog post there are some gems in there that will help you out. Then reply here or pm me with any questions.