Oh, my heart is breaking for you!!!

Four years ago my sweet sweet little pug Kinky was to the point where she could barely walk as well, and had a hunchback and was getting bald and incontinent. Like you, I knew I HAD to help her out of this world too. Kinky looked JUST like your Winston.
It was so hard, taking her on her last little walk in the tall grass, letting her smell everything as much as she wanted, then going to the vet's. It's so heartbreaking. Something about pugs- they are so sweet and loving, never a bad thought in their little walnut brains all their lives.
Your little Winston was a LUCKY LUCKY puglet to have such a loving mommy as you.
Sending comforting vibes your way, may you find the strength to deal with this. xoxox