I'm soooo not an expert on this, so what I have to offer up is only a random thought, but here goes fwiw! I have one child who struggles with anxiety, and another child who is possibly the least anxious person who ever walked the planet. My dd who struggles with anxiety will get caught up in the "fair vs unfair" mindset quite a bit, whereas my non-anxious dd will also get caught up in it *occasionally* and typically when things are causing her a bit of stress. So I would look at the situation with your dd and ask - does your dd's extreme sense of fairness/etc crop up in stressful situations or is it present when all is calm and everything's a-ok? If it seems to be related to higher stress situations, then perhaps you can deal with it by helping her think through the situation, what's tipping off the stress, help her figure out a way to diffuse the stress etc.
I realize it can also be a circular thing - stress leads to anxiety leads to heightened sense of fair/not-fair etc, leads to more stress.
I also might consider short-term counseling for your dd too - we've found that short-term counseling can be very helpful for specific types of situations like this.
Best wishes,
polarbear
