Had to put mine down several weeks ago. She was also 18. I lost several other pets and four grandparents without shedding a tear, but this one broke me.
The vet appointment was at 6pm, so I could spend the last day with her. She acted more or less normal for her age, except not eating for three days and throwing up water.
I was rubbing her head until her heart stopped. I still miss her so much.