When you are attempting to do something that you've previously failed at, like a workout routine or writing a book, the past will always haunt you. The troll inside your conscience will constantly recall that you tried this before. It didn't work then and it won't work now.
Much like Thomas Edison, the first, second, and even hundredth attempt may not be the one that results in a working light bulb. It might be the very next try that finally succeeds. If you knew which attempt was going to work, even if it was 50 away, would give you encouragement. But it's the unknown that makes it unbearable.
For encouragement, do something you didn't do the last time. Try something dramatically different that will make this effort brand new. Workout on a Sunday, get up at 5am, write 100 words before breakfast, something, anything that you didn't do before. Just the knowledge that you're doing something different, no matter how small, may encourage you to try again.