Poo.

First up, relax. Chill out a bit. This has been a long road, you've done all the right things, and at the end of the day you need to remember that she won't be wearing a nappy at her 21st. Forget about her age, or school, or anything. Let it go. Take the pressure off. Off you, off her.

Second, look at the rest of her life. The non-poo stuff. I have seen (childcare) lots of kids go into regressions for all kinds of random reasons, and really needing some help to come out of it. Is there any source of stress in her life? Even getting the hurry along to finish breakfast, or being told she can't do XYZ because of some reason that is quite logical and rational to grown ups but ridiculous to her. It could be something really little. So do some emotional nurturing, not related to poo.

Get her on a probiotic. It can't hurt It's my answer to everything poo related!

I think I had more ideas, but I forget. You're doing a great job, and this will all end up okay eventually. Good luck!