Feeling guilty after having my dog put to sleep

I had my 9 year old Pomeranian put to sleep yesterday. He had heart failure and lung disease and I couldn’t bear to see him suffer anymore. The vet put him in straight into an oxygen chamber as he was struggling to breathe. I decided to stand next to the glass chamber rather than hold him whilst I waited for my partner to arrive. He looked happy and healthy as he could finally breathe easily with the oxygen. When my partner arrived we took him out of the chamber and held him one last time while he was put to sleep. I have been feeling guilty ever since, wondering if I should have held him for those last 20 minutes instead of keeping him in the chamber. I keep thinking everything over in my head, wondering if there was more I should have done and if I made the right decision. He became distant in his final days and wanted to be left alone, seeing him in the chamber was the first time I had seen him happy. I just wish I had hugged him tighter or showed him that I loved him more. I hope he visits me one day in my dreams or gives me a sign that he is okay. I never realised the pain of knowing I will never see him again.