Sad about my cat

I got all teary today thinking about our old Callie cat. She's getting on in years - at least 18 years old. She's a beautiful thing, soft fur, nice purr, really gentle. In the last year or so she's really shown her age. Meowing loudly for no apparent reason, in the middle of the night sometimes. It wakes us up from our sleep. She protests very loudly when anyone picks her up or moves her too "roughly". She walks with her tail down and head down, slow. She still is able to jump onto the chairs though which is good for her because they provide a nice bed for her. Lots of times she will fall asleep with her head up and very slowly it will relax so that her nose is pointing straight down onto whatever she's fallen asleep on. It looks really strange. I think she has some pain, not sure how much, but they say that cats tolerate pain really well and don't show it until they are in agony. I prayed tonight and asked God to help her fall asleep forever very soon. I am sad for her and don't like her to suffer and the thought of having to drive her to get a shot of pento would be so scary for her. I just want her to go peacefully and knowing she was loved.