Try and eat some juicy mango =P, in all seriousness, you remind me exactly of myself when I first became sexually active with women. When with women I feel this strange urgency to perform well and need constant stimulation to stay hard, I haven't been with many guys, only 1 close friend...but there was no preasure and I was hard before my pants even came out and stayed hard even after cumming.

Just relax, lay back and let the good feeling flow, and most importantly make sure your GF knows its nerves causing that and her making assumptions that you don't think shes attractive will only upset you more. Theres no magic thing you can do that'll instantly fix this problem, its only when you become comfortable with the person that you'll stay rock hard all the time.