Kaz, Bill was the same. There were days after he’d been diagnosed that we were discussing having to call the vet because we couldn’t see him suffering any longer.
Honestly not sure how it works with the Cushings meds but once his insulin dosage was tweaked to the right amount he was back to his normal self. He’d have to spend a day at the vets with hourly blood glucose tests so that they could chart what they call a curve and see how his body was reacting to insulin. If the curve wasn’t following the expected path then the dosage would be changed.
When it comes to injections I definitely recommend a Vetpen, it’s far less faff both for you and Fraggle than a syringe, and if you do go down the Vetpen route then let me know and I’ll send you a link to buy compatible needles at a fraction of the branded ones.