Duncak, I am so sorry to read this. I am literally crying as I write this. It is extraordinarily hard. Especially when they are young and seemingly in their prime (like my Roxy was).
The only things that help are time, talking, and exercise/fresh air. We build our lives around our pugs and their routines. And it can almost feel like a betrayal to change that. But it isn’t a betrayal. It is something you need to do for yourself.
Pugs are notorious for being susceptible to anesthesia so do not blame yourself. This does happen. You did not kill Ludwig. And I’m pretty damned sure he knew you loved (and continue to love) him very much.
I’d give you a big hug if I could because I know exactly what you’re going through right now and it hurts like hell. But hang in there, get out, hang with friends, do things because while you’ll never forget nor stop loving Ludwig, life will get better with time.