I'm a veterinarian and I deal with this type of situation quite often. I'm so sorry to hear of Cassie's troubles tlay. The advice I generally give to people, is that they will know when the quality of life of their pet has diminished to a point where euthanasia is the best option. The key tlay is YOU will know, and I'm sure you will always do the right thing by Cassie. You cannot put a price on those few extra precious days you have with her, but if/when she is truly suffering, then she would probably want you to let her go. It is one of the most heart wrenching things we as humans have to do for our beloved pets, and never easy, but ultimately it may be your final act of kindness for Cassie. Good luck and prayers to both of you my friend.
BCD