First thing that comes to my mind is a reward system. I know people don't like reward charts, but they've worked well for us on the odd occasion we've tried them. I also find reward charts remind ME to praise when they've done the right thing LOL.

Would she respond to collecting stars or something for each time she changes herself straight away, and maybe extra stars for a completely dry day?

My 5yo was hopeless at getting herself to the toilet at our last camping trip.. drove me mental. Hers was purely being too busy and leaving it too long to walk up to the facilities (and even too long to rip off her pants and go behind a tree). Maybe it's the age where they think they've got this toilet thing down pat, so think they can hold longer than they really can :/

I hope others have got more suggestions and it improves somewhat before school starts. And don't beat yourself up about losing it at her, they're at an age that we expect them to understand these things, and it's very frustrating!