Ideas for a couple of fanfics keep circulating in my brain. Problem is, I want to work on an unrelated novel and my brain keeps getting distracted by and focusing on these fanfic ideas, leaving me mostly unable to think of said novel. What can I do?

Assuming you haven't already, maybe just take a break from your main project to draft some ideas down for later. As a comparison, sometimes when I'm bored of working on my main art piece I sketch an outline to color later to sate myself and get me going again.