Hi Mitchie,
I typed a reply earlier and then didn't post is as really don't want to upset you but now think maybe I should just tell you..I had a horse, who sounds similar to your pony, I adored him, he was a shit, still is! Was very unpredictable and often dumped me, I tried all sorts with him, had some top riders come and school him and they could get some lovely results from him but he would still be naughty even with them on board. I wold get a "better" rider to school him before I got on, put him on the horsewalker, lunge him anything to try and get the fizz out, nothing worked 100%. If he was in the mood to be good he would be and he and I had many wins when he was in that good mood, if he was in a crap mood boy would I know it. There were no triggers, just how he felt like behaving. I eventually, after 3 very long years of trying all I could, gave him to my best friend, who is a talented rider, she still has him (7 years later) and he is exactly the same, she can have fantastic seasons on him eventing over huge courses and then another day he will dump her in front of a cross pole! She even hacks him out with a neck strap as he is even happy to dump her on the road.
Some horse/ponies are little shits and neither schooling nor age will change that (the horse I'm talking about is 14yrs old now).
If it were my DD I think I would impose the "you can't ride him at the moment" rule and hope that she loses that bond with him or he turns a corner and becomes the pony she needs. I hope he does and if he doesn't at least you can try and get your DD to love something else instead.
I now have a fab horse who does exactly what I want her to do (she's a bit cheeky but never mean) and am so glad I parted ways with the handsome handful!
Good luck :-)