To ease it on you a little, I have a senior cat too and she's half of my world, I wanted someone who'd relate and tell me how to deal with those feelings, I posted this matter on Reddit, and one comment that stuck with me is "love her to your fullest, make her world complete and full of love and comfort every day she gets up to live it,think about today and leave tomorrow to deal with itself"
So instead of this about what's going to happen tomorrow, love her today! Cats can feel their owner's feelings UK so Ur only making her sad! Good luck<3

Confession: i long to be loved.
Your turn.