I don't know what to say either except I am so very sorry.
We were in a similar situation with our Westie several years ago. She was fine when I went to work. Within 30 minutes my husband rang to say she was crying in pain and he was off to the vet.
To cut a long story short, she had an inoperable tumour and we were advised to have her PTS that afternoon.
Although it was terrible and a dreadful shock to us all, we all had the opportunity to hold her and say our goodbyes (including my son who had been brought up with her and never remembered a time when she wasn't around)
My husband and I stayed with her, and she was happy, comfortable and still wagging her tail to the very end - so although it was horrible, heartbreaking, sad and everything else you can think of for us - for her, she was with those she loved, being cuddled and told how much we loved her as she went so she didn't know any more pain (she had pain killers during her last day so she was comfortable)
Less than a year later, our yorkie was taken ill and was at the vet and ended up not making it - so she died on her own at the vets during the night.
I know which end I would prefer for a dog I loved so take some little shred of comfort that you can make your dogs last days happy and full of love - and no pain - and then when the end comes you can be there for him.
My thoughts and prayers are with you and your family at this terrible time.
