Jerusha Cowless, agony aunt: "Do I go with my head and finish the book that I think has a good chance? Or go with my heart and write the stuff I really enjoy?

Dear Jerusha - I'm currently working on my WIP, a thriller/drama which I think is a really good idea and I love the plot. However, I'm struggling to get motivated with it. But after churning out a few short stories...