Imagining a specific failure, bringing it to a near mode, can remove the fear of unknown. Without fear, it is easier to work. On the other hand, imagining a failure is priming oneself to fail.
Which influence is stronger?
Maybe the result depends on how exactly, and how long is one imagining things. Methods similar on surface could bring different results.
Usually failure is a lot less terrible in reality than what you imagine.. or when it is terrible you tend to very prepared for your task. If you have too much optimism you don't tend to be prepared enough and when you fail it is hard to try again. Being realistic and basing your life on reality is much more powerful than being an optimistic wishful thinker.
Deep down your true self knows you are trying to deceive it.
And remember what you can really accomplish if you just progress one little step at a time..
the video
I'm posting this because akrasia, procrastination and willpower are often discussed on LW. I haven't read the book, but for those that are interested "The Willpower Instinct" and "Maximum Willpower" are, from what I can tell, exactly the same books.