I'm writing a book, but I'm having a hard time pinning down what genre and subgenre it belongs in. If I can figure that out, I can make small alterations to the plot to make it better fit in it. For starters, it is both a romance and a fantasy and a mystery. So help me brainstorm ideas for where it could fit and slight alterations I could make to make it better fit in that idea. 
That does sound hard. Here are a few ideas for you:

- Fantasy Mystery - You could highlight the mystery and make it the most important part of the story so that it fits in the genre and have the fantasy as the subgenre by only barely mentioning the romance. 
- Fantasy Romance - You could also focus on the romance, putting it in that genre and in the fantasy subgenre. The mystery could be wide open from the beginning as something the reader knows but the characters don't. 
- Mystery Romance - If you downplay any fantasy elements and focus on the romance and the mystery, this category would work perfectly.  

I hope this helps with your book!