try not to worry
I am in complete agreement with Oliver and quila. If a woman is truly interested in you then sex, in general, is only a part of that package and if their interest is hinged entirely on sex then quite frankly you are better off without such a person in your life.
Since sex and ED is the issue of concern then it is important to develop your oral skills and use the rest of your body to show her your passion for her. Attention to detail is important - discover (or if you already know them, use) her erogenous zones. I have gotten girlfriends and my wife off before by proper breast stimulation and using her erogenous zones (above the waist line). Take your time and use all the other gifts you've been given to please her. Research tantric massage, erogenous zones, use toys on the occasions when your body wont perform and, most importantly, trust in your relationship and your love for each other.
Give yourself some credit too, it takes courage to address this situation within yourself and being willing to move into a poly relationship as well. There are alot of men out there who allow such an issue to wreck their life without seeking advise or solution. The fact is you aren't at fault for this and you shouldn't allow that illness, or its effects, to rule your life or dominate your self confidence. I agree that having a discussion with your girlfriend is the best first step.