The problem causing a lack of motivation and willpower is that people make decisions incompletely and without organization, and are thus ruled by their subconsciousness. Most of their thinking is responses to what is outside them: to written words, images, and others speaking. Think of how much writing there is on the internet, so many debates and petty arguments, yet there is so very little directed at one's self, it is always for others. One must learn to communicate with themselves.
The tool to do this is written language, which is without doubt the most important invention in all of human history. With language transient and fuzzy thoughts become concrete, they can be inspected and re-arranged. Pen and paper can expand the power of one's mind more than any other tool, which is so true that we take it for granted: how many numbers or words can you hold in your memory compared to writing them down, and how easily? Writing makes informational storage trivial.
So the method involves writing to yourself to change behavior and action. I recommend using pen and paper for this, as it is much more physical than writing words on a computer screen.
With the power of your own words you will be convincing yourself to act, feel, and think exactly as you direct, literally writing your own victory. The premise is that it is impossible to not act on something you have fully decided is reasonable to do. The problem is that often our decisions are not "fully decided," they're in a disorganized haze in our minds, pushed and pulled by subconscious drives and desires.
Think of a problem you have, such as a fear, feeling unmotivated to do something, procrastination, etc and write it down. From here on out you will be writing out all of your thoughts on paper and reviewing them as you go. Start with a small problem, such as needing to exercise or do housekeeping. Now ask yourself questions such as "What is causing this problem?" and "what can I do to help solve this problem?"
Now try to answer these questions as fully as possible. Be honest with yourself and how you truly feel and think about them.
Next, think of a range of possible solutions that can solve the problem either fully or partially. List pros and cons and each solution, and again be honest with personal cons. Narrow down solutions to a single one you can do immediately, or set an exact time to do it if you cannot, using a cell phone timer to remind yourself.
If this process has worked you will put the pen and paper down and immediately do what you have willed yourself to do without a single trace of reluctance: you have fully consciously decided to do something and forced your subconscious to comply. If you still feel reluctance, write down that you feel reluctant and explore the reasons why, and argue against those reasons - don't give your subconscious a single out, a single reason to resist.