I want to say (from personal experience) it takes quite a while for them to mature mentally. Like it took 6 years for my qh. My paint is coming 7 and I only now feel she's kinda "coming there" in her mind. While I did start both of them rather young I started serious lessons only last year (when they both turned 6). Have you tried trail riding her may be to keep her curious and going?
"Life isn't about waiting for the storm to pass: it's about learning to dance in the rain..."
"When we are no longer able to change a situation - we are challenged to change ourselves."
"How people treat you is their karma; how you react is yours."