I said goodbye to my Rexie when he was almost 16 too - he had cancer and one day, it was his time. You just "know" when they aren't happy, don't want to be here anymore. And somehow, although it's sad, it's peaceful for them.

When he was old he'd sleep a lot, get grumpy at the pup, and not leave my side.
When it was time, he had a bit of a play but that only lasted 5 mins.

If she's having trouble jumping up, why not do what we did, and make "steps" for her to climb onto the couch?

Good luck, and enjoy her love