I am so very sorry for you. What a terrible decision to have to make.
I don't know if this will help or not, but I will just share two stories that we have endured over the past 2 1/2 years.
Scuffy our Westie was taken very very ill, very suddenly. In the morning she was fine, then she suddenly began to scream in pain and my husband rushed her to the vets for tests. By lunchtime we were told she had an inoperable tumour and was in severe pain (although they gave her painkillers at the vets) We made the decision to have her PTS and were with her that afternoon when she went to the bridge. We were all totally devastated.
A year later, Meg our Yorkie was unwell. She had been battling illness for a while and getting progressively worse. She picked up a virus and was taken to the vet a very very poorly dog. She battled for 3 days and nights to get better, and the vet did everything they could. There was one more treatment they said they could try which MIGHT work. After a long discussion, we decided we owed it to Meg to give her this one last chance, so we asked the vet to try on the understanding that if it did not help, we would go back the following day to be with her whilst she was PTS.
We had a call the next morning to say Meg had died in the night. Once again we were devastated - BUT
it was much much worse because she had been alone (and maybe scared and in pain) when she went to the Bridge.
Scruffy had been held in our arms (am crying now as I remember both) and told how very much we all loved her and would always love her. That gave us huge comfort afterwards, and I will always remember it as being a lovely and peaceful experience - although devastating.
What I am trying rather crassly to share with you is that IMHO it is better for the dog and for you to take the awful decision and let them go quietly and calmly, knowing they are loved, than to leave them to suffer and possibly go alone.
Whatever you do, Molly will know you love her and I know you will find the strength from somewhere to make the right decision for her.
We are there for you when you need us to be. COL comforted me hugely when we lost Meg and you know you can log on anytime day or night for help and advice. I wish you luck and strength, and send you lots of hug and positive thoughts.