Cat weeing everywhere

[Thatsalovelycuppatea]

Evening all.

I was wondering if anyone has any tips.
I have an old cat who has regular cystitis. I've only recently taken her to the vets for continually peeing in the house.
It started off in the corner of one room and now she is going under coffee tables etc etc.
She has had two injections at her last visit and I'm literally loosing my mind what to do now. She seems generally with it as in personality but I cannot carry on like this and having her ruin my house. She is to old to rehome and I am wondering if there is anything I can purchase to prevent her from weeing. Thank you.

What food does she eat?
How many litter trays does she have?
Do you use a regular treatment like cystophan to reduce urinary problems?

I normally get her food from the co op as it's alot cheaper than other places.
She has had medication but not worked, so had two injections.
The vet thinks it's mainly behavioural issues but I don't agree because I know once a cat goes in one place, they don't stop. And it's an old cat with cystitis.
We have a tray in the kitchen which she uses. I'm reluctant to put any in living room due to the dog.