Some cats are scared of Mirrors

My cat is about 12 years old now and it seems like she’s rediscovered the mirror. She used to just ignore it when she was little, but now whenever she sees herself in a mirror, glass, window!, she goes crazy. My cat hates other cats, and she thinks her reflection is another cat. She goes insane, I’m scared for her. She’s already pretty old, this can’t be good for her. You can’t even touch her because she’ll scratch you. She’s too caught up attacking and hissing at the mirror that she doesn’t realize it’s her own reflection. Now that she thinks there’s another cat in the house she marks her territory everywhere! What I can do?