Kimmi,
The reason you're being asked to find comparative titles is for potential publishers to see where your title might fit into the market. Absolutely, positively DO NOT say that there are not comparable titles. that means there is no market for your book. Rather, take some of the more well-known books that might be kind of like yours (yours is a memoir, right? so of course there are other memoirs out there, and very likely on your topic, as well). Then talk about why those books are different from yours. You don't need to bad mouth them (probably not a good idea, even if you want to), but just explain why that market has space for another good book (yours).
Good luck!