Chances are, if you are anything like me, you have an internal chatter going on all the time about your work. It is important to monitor this chatter because it is giving you useful information.
Listen, first of all, for positive messages. What are you telling yourself about your project? What metaphors are you using? For example, I found myself repeating the phrase "seamless whole" over and over again. Conceiving my project as a single long essay was motivating for myself.
Vague positive messages like "I can do this" are less helpful, I've found. They are not actively harmful, but they aren't as motivating as very specific metaphors or images. By the same token, vaguely negative messages should be brought to the fore and actively refuted. I'm thinking of things like "I don't know enough to write such a book" or "I'm not smart enough" or "I'm a lazy person." We all have those thoughts and they can be very damaging.
Very specific negative chatter, in contrast, can be helpful in clarifying specific problems in the substance of the work itself. If you keep hearing that voice in your head say it's not satisfied with your contextualization of Valente in regards to counter-reformation theology, that's significant information that you can use.
Then there is the chatter that actually contains ideas that you can write down. This is not meta-chatter, but actual thinking about the project that you should always be listening to.
So monitor your internal voices and filter out the destructive ones, using the useful information for your own benefit.