Oh Robbie, I just know what you are going through.
Honey, the cocker from next door who I have looked after for a long time whilst her mum was at work, had a growth on her lady bits which kept growing. She was on medication for months but it still kept growing. We knew the vet would not give her any more medication when he saw her, growths were starting to break out all over her, so we knew what would happen. But Honey was not poorly in any way, so I know what you mean about Charley. Honey would eat anything you put in front of her and squealed with excitement when she saw her lead. The evening we went to the vets for the last time she was so excited about going in the car, had a good sniff around the vets car park and went into the vets quite happily. It sounds awful but her mum & me were almost wishing for her to become poorly before the dreaded appointment, as we felt it would have made it easier.
I will be honest with you, it nearly broke us, her mum and me, but I can honestly say that there has never been one minute when either of us have thought that we did the wrong thing for Honey. We know it was the medication (steroids) that was keeping her well and we knew she could not carry on the way she was, it wouldn't have been fair on her and we would have been keeping her with us for us not for her.
My heart goes out to you, I know just how you are feeling and I feel so sorry for you.