Hi Rachel, your book sounds great! I don't have any experience writing query letters so my advice is purely from a reader's perspective.
The first paragraph raised so many questions as to be confusing. An assistant psychologist discovers that magic is real, which makes her job even more difficult. Wouldn't magic make things easier? What makes it extra hard, is battling the same ancient darkness as one of her patients. What's the ancient darkness, how does she battle(with magic?), and is she battling it together with her patient?
The next paragraph is where I got interested, but I really sat up and took notice when you mentioned the Reality Filter. As Laure said, the beginning is nice but nothing new. Could you start here instead? (I loved this whole part!)
The world as we know it has been subject to a Reality Filter. Kelly, an assistant psychologist, has just smashed her own filter to bits and there is no going back. She can now see and hear things that are Really There, including three imps, who’ve decided to befriend her. This is extremely distracting, especially when she is trying to support her patients, who see and hear things that AREN’T really there, or so she thinks.
The last para is good but vague, you can flesh it out now that you have more wiggle room. Kelly’s discovery takes a dangerous turn when she learns that one of her patients is plagued by an ancient being made of shadow, and it is up to her to do something about it. Why is it up to her? Why would she take this risk?
Just some ideas for you to think about. Good luck subbing!