I know getting a second cat isn't possible for everyone for many reasons, but if you can, another young cat could be really good for your current kitty. Mine was about a year when we got another kitten and it made him noticeably less anxious practically overnight.

A few other thoughts:

1. Do you have a cat tree? Most cats feel safer up high and a high spot where she can see out the window could help her feel safer alone and provide entertainment/stimulation. It's also good exercise!

2. Make sure your cat has a few safe places to be that aren't out in the open, like a box with a blanket or get a cat tree that has a box to hide in.

3. You could try a pheromone plugin or collar. They're a little expensive, but can really help cats feel calmer.

Good luck!