Dawn, if it didn't break your heart or you got upset about it, then I'd say its time to hang up your boots!
I know there are those who can make these decisions without feeling or emotion but I, like you, struggle with certain decisions that although my head knows its the right thing to do, my heart still tugs.
In this instance though I do feel it is one that has to be made with your head based on the information from Sean or the other vet as to how far along she is and what their recommendations are to ensure the best for mum. As hard as it is a decision to make mum has been through a lot and whilst yes she'd go through it again as she has no choice it may not necessarily be what is best in the end. Either way the vets will guide you through it based on their opinions and if you follow their advice then it does take it off your shoulders in a way regarding the decision.
If you need or want to talk it through, you know where I am!