I feel so bad for you. It's nothing short of horrific to make the difficult choice to ease a beloved companion across the 'Rainbow Bridge' where they will no longer be in pain. We love them so much that we want to hold onto them, but there comes a point when it's just pure selfishness to NOT let them go. I know this won't make you feel better right now, but letting Winston go was an act of love on you and your husband's part.
We had to make this decision 2 years ago about our sweet boy Roux. He wasn't a baby, but we had only had him for 6 years. He was a rescue from an abuse situation whom we adopted when he was 4 years old - an English Mastiff who was so malnourished that he only weighed 80 pounds when he came to us...and he was bald and covered in sores. He was scared of his own shadow and would cower when we raised our hands to toss a ball for him to chase. Even the microwave beeping would make him put his tail between his legs and curl up into a ball. We took in a 4-week old rescue pit-bull puppy the week after Roux came to live with us, and she made a world of difference to him. He had a friend that he wasn't terrified of (our cats are kind of *******s) and she helped him come to trust us. He and Nola were inseparable for the 6 years that we were blessed to have this sweet boy in our lives. Roux eventually got to a healthy weight of 200+ pounds and became a huge love-bug - total 'velcro' dog
When we had to let him go just a couple of months shy of his 11th birthday, it was one of the most painful things I've ever experienced. It was almost as bad as losing my brother to a heart attack the year before...not sure if this makes me a bad person, but whatever.
But the sweet part was that when we took Roux to the vet's office to ease him out of this world, they came out and gave him the injection in our truck (which was a 'safe space' for him), and Roux's favorite vet tech Jim came in on his day off to be with him at the end. The thing you've gotta know about Jim is that anytime we wanted to get Roux in the car/truck, all we had to say was "You wanna go see Jim?" and he was inside in a flash - he adored himself some Jim! So Roux was surrounded by the people he loved and we all had our hands on him, comforting him, when his heart took it's final beat.
Our pitbull Nola was inconsolable, but one of 15 year old ******* cats took pity on her and has stuck to her like glue ever since...he snuggles with her, shares a food dish with her, and they groom each other all the time. I've never seen anything like it.