You should be psyched, dude! The fact that similar novels have been sold means there is something of a market for the story you want to sell.
My suggestion is to buy Lauren Kate's books, read them, and be sure yours aren't too similar to hers. I'm working on a novel about the life of a certain Egyptian queen, and to be sure I didn't get too close to another novel about her life, I bought it, read it, and took notes so I could avoid undue similarities.
Listen: Paranormal books are huge, huge, huge right now. Huger than you may realize. I didn't even realize just how huge until I took a part-time job at a book store, and oh my god, I stock the paranormal paperbacks twice a day because they fly off the shelves that fast. I'm not kidding. Twice a day.
There are probably other YA paranormal novels about girls and hot angel dudes. You just haven't found them yet. You shouldn't let that discourage you, though. Yours will be different. It has to be, because you're writing it, and you're not Lauren Kate or any of the other authors who are tinkering with this subgenre. Merely having a love triangle between a mortal girl and angel guys isn't enough to make people cry foul on you. Do you know how many teen vampire love stories are out there? Oi!
Write the best book you know how to write. Read as many other similar ones as you can, so you can avoid too-close similarities. But don't sweat it too hard. There will always be another book like yours out there, somewhere. There is nothing new under the sun, etc. Just worry about doing the best job you can do, be grateful there's already an established market for the work you want to do, and write, write, write. 'Cause it'll definitely never make it to a book shelf if you don't finish it.
It's just frustrating, especially when you receive rejections on your queries. Ugg.
