I struggled for many years of lack of motivation. One source of insight into motivation is "Drive: The surprising truth..." http://www.amazon.com/Drive-Surprising-Truth-About-Motivates...

The main takeaway is that you want internal motivation, not external!

I found my internal motivation through finding my "why", read "Start with why" for an explanation http://www.amazon.com/Start-Why-Leaders-Inspire-Everyone/dp/...