I can relate to this. I have days, even a week at a time where I am able to cope well in situations that usually have me in a blind panic. I feel soooo confident and positive. Then I'll have a big, full-blown attack (usually when I'm already stressed or not feeling well) and I think "aaaaahhhh", I'm back to square one.
To feel better about it, though, I try to focus on how many good experiences I have. Over the past few months, I have been able to do things I couldn't do earlier this year. I find I'm going to do much better in situations if I focus on the "I can do it" feelings, rather than "Oh, no! What if I panic". Sooooo...to make a long story short (too late, I know! haha), I guess it really does help to think positively, rather than worry about having a relapse (sure, easier said than done!)