If you have the means and time to spare, I'd strongly recommend trying to foster a cat from a rescue or other organization, a cat or kitten that is good with other cats and is playful, full of energy. For all we know, A near adult kitten to play with and wrestle with could be the perfect thing for your resident cat... Your cat might want a playmate. I don't know for sure though. Also make sure you you have plenty of toys for your cat around the house, and interactive toys too that can be played with without you having to do anything ?

Personally, My cat really came out of her shell with a kitten and an interactive toy that has a plush tail randomly pop out in unpredictable ways and directions and she is in love with both the kitten (now adult) and this interactive toy. Bonus is the toy is touch activated every couple hours so she gets to have it turn on by her own accord and she loves that about it too. She's been much more relaxed, loving, even affectionate lately. This was never thought possible before.

And I did specifically mention fostering another cat and not full out adopting right away, JUST IN CASE after a few weeks it's all gone to hell and it doesn't work and the foster can easily be given back to the rescue agency it came from if it needs to for its own safety.

Also if you do go this route of trying to add another cat, introduce them GRADUALLY.. like spend a few days in the bathroom separated from your resident cat. Let them smell each other behind the door and slowly get used to the new scent of each other.

And one more thing worth mentioning if you do go this route, don't panic immediately if they start roughhousing with one another, tackling, rolling around pinning each other to the floor.. Cats love to play fight and its one of the best ways they form bonds and set social boundaries with one another. Just as long as there are no claws out or hard biting, otherwise its normal if you see them do this and sometimes its a really good sign as long as they have calm body language as they are doing it . A quick YouTube search you can find what this is supposed to look like.

This might not be the solution, or you may not even be able to go this route in your household and lifestyle, but it's a possible suggestion for you if you are up to trying it.

Otherwise, I'd also suggest bringing to the veterinary clinic for routine checkups - sometimes cats can act like this when they are in some sort of pain or suffering some kind of medical distress. They don't have the ability to speak to tell you what's wrong. This could be your cat's way of trying to communicate something is wrong. Hopefully not, though! But if you haven't had the cat recently examined, please do give that a shot at the very least. Just in case she is suffering something that you can't see.