Similarities to other works
Question: What do you do when you come up with an idea and plot for a novel, and it turns out that another author has already written something very similar? Answer:
You change something.
This is how genres are created.
Imagine the writer who invented the first love story involving a romantic triangle. (It might have been Homer's story about Paris, Helen, and Menelaus. I'm not sure off-hand.)
Another writer then comes along and thinks, "I like that story, but wouldn't it be interesting if..." Then he/she writes a story with a twist on that idea.
Thousands of years later, writers are still writing fresh, original stories about romantic love triangles, and readers are having fun reading them and trying to decide if they are Team Peeta or Team Gale, Team Edward or Team Jacob.
And romance has become the biggest selling genre.
Genres also come in waves. For instance, when Harry Potter
looked certain to become a huge publishing success, publishers started looking for other urban fantasy YA books to print so they could take advantage of the craze. So many books about teenage vampires, werewolves, and other fantasy creatures came out, each one offering their own twist on the core idea.
Your story doesn't have to be entirely original. There's nothing wrong with writing within a particular genre. You just need to put your own twist on an existing idea. Perhaps you add an element that you haven't seen before, or you take away something, or you change one element into something else. Maybe you cast a very different person in the role of main character, or you change some of the circumstances, etc.
Once you have a new twist, that will dictate other changes. The result should be a story idea that seems fresh and exciting. If not, try again.