Have you tried NutureCalm? It's a calming collar that she wears continuously for a month at a time and as a cat lady with a chronically anxious cat myself I've found it works much better than the diffusers since it's on her constantly and releasing the scent directly into her senses.

https://www.amazon.com/NurtureCALM-Feline-Calming-Pheromone-Collar/dp/B00IUJDROY

It's not a perfect end all be all product, my boy still has his fears, but he's been much better with immediate concerns and stress and outside of changing his environment as much as I humanely can, this has been our most natural and non-medicated option that works for us.

If you decide to try it, don't get it from your vet...they charge twice as much for it than amazon or chewy.com.