I understand why you are so upset.
I have the impression that there is a link between the strike your penis is on and the feelings you have for this woman. The fact that you were able to function with other women who you didn't care so much about, and that you can't function with a woman who you do care so much for is significant.
Also notice the language that you use to describe your feelings for her. It's as if you are reluctant to admit that you love her. You sound hesitant. Then consider what's happening with your penis. There is a serious hesitation!
You will be on the path to solving your erectile problem when you get in touch with why you are afraid to love. Think of it this way, your penis is now sending out smoke signals, symbolic communication, if you will. When you get in touch with your feelings on this subject and begin to work them through, your penis won't need to send out danger flares.
So figure out why love is scary. Read through my Advice Archives for ideas, which may range from fear of intimacy, commitment, abandonment, and so on. I promise that this problem can be healed. But make sure that you don't add insult to injury!
I want you to also chill out about this temporary problem. If you get your shorts in a knot, your penis will malfunction simply because the chemicals associated with worry are incompatible with the chemicals required to become erect.
So, you will need to tell yourself, and your girlfriend will also need to tell you, it's all right if you don't get hard. Read my Archives on this subject as well. And explain to her that worrying about whether you get hard, and worrying about whether she will be upset is enough to get you down and keep you down.
Keep me posted.