My best friend used to have a magnificent Standard Poodle. He was tall, for a Poodle, and regal as all get up. He had been aging, but nothing untoward. Then one night, he woke up with bloat, we were both asleep, but he banged an aluminium step stool against my wood stove. When that didn’t quite get me up, he did it again. It took but one look, to know something was wrong. Minutes later we were driving towards a 24/7 emergency pet hospital. When we arrived, he got out of the car, and walked towards the entrance. Within a half hour, the options were on the table, and none of them were good. He was what they called, put down. As they filled the syringe with the deadification, he looked up. Just patted him, stroked him, and he gradually without any visible pain, went to sleep.
It is an agonizing experience, to know what is to be, and to know that no matter how one feels, all one can do is look, and try to ease the pain or the fear.
Worry not for feeling sad, you have lost a friend. That is enough to make anyone sad. Remember, when it is time, that Sophie also enjoyed being with you, your smile, and your times together. She would want you to be happy.
Having a physical form, is having something that will wear out, not quite like erosion, but close. Welcome the time you had, the memories you have, and warmth it will bring you over time.