We went through this with China and osteo. We didn't know she was sick. I touched a bump on her shoulder and she screamed. This was around 7:30 in the morning. We rushed her to the vet. X-rays showed that two of her ribs were broken from the osteo. She was gone by 9:30.
We were numb from shock, then guilt, then a myriad of emotions that we still feel 7 years later. Shock because we weren't expecting it, then guilt because our girl was sick and we didn't recognize it, and then the thoughts that roll around and around in your head for days, especially when you are trying to go to sleep.
Missy, the pug, was going downhill for a long time and we knew we would have to let her go. When we made the decision, it broke our hearts. We knew we had done what we could to make her comfortable during her last months, but we still kept second-guessing as to whether we should have let her go sooner or tried to help her for a few more days/week/months.
I don't think there is a pat answer to your question. I just know that many of us have been there and our hearts share your pain.