Oh dear love you, it's heartbreaking. We lost one of ours just after surgery, she died in the vets without us. After that we had our boy pts after a sudden cancer diagnosis, and it happened really quickly. He had been getting old and creaky, and then it suddenly became very obvious there was a big problem. He went in for tests and we discovered he was riddled with cancer. We had a little time with him, but he was out of it on meds, and I asked for him just to be allowed to go.
You made your choice for Carlo because you love him and put his needs before yours. I completely understand your pain and feeling of loss. You are not alone in this, many of us have been down this road, and there is no easy answer. Please do not reproach yourself, you say that you saw the change in Carlo, and that is why you changed your plans. It was right for him, even though it pains you, a hard choice for the right reason. Please take care of yourself.