It must be really difficult for her to see you suffering and not know how to help. Does she know that when you can't go out with her, it's not because you don't want to be with her but because you're truly not feeling well? Do you think, since she doesn't understand the anxiety part of IBS, that she may feel you just don't want to do the things she likes? There was a period where I didn't want to go to the movies because I was afraid of having an anxiety attack in the theater--and my boyfriend loves going out to movies. I could see where he might have thought I didn't like his choice of movie or just going to the movies in general. Maybe she needs to be reassured it isn't her and that just by being with you and loving you she's already helping you a great deal.
I think it's good that you can talk about it with her and she admits it's a difficult situation for her. Your openness and honesty shows how strong you really are. I know it was really hard for me to talk to my friends, family, and even my boyfriend about IBS and anxiety. I think it was a mix of pride and embarrassment that kept me from sharing. But once I explained what was wrong and how I knew it was affecting our lives, my boyfriend turned out to be really supportive and extremely patient. (I would like to think I'd be the same if roles were reversed, but who knows?) I think the good thing about IBS, if you can find anything good about it, is that it makes you a more compassionate person.
Please keep us updated on your progress with the diet and the AD, if you decide to take one.