Think of it this way:
Your will is like a stering wheel in a car. It directs you to where you want to go. It does not power the car, it does not make anything happen. The power that makes the car go is the energy harnested fromt the combustion engine turning gasoline into flame into power.
In the magickal sense, gasoline is like energy, which can come from anywhere(so yes in a way from yourself as well). The problem with self energy is that you'll wear yourself out quite fast, hense the reason for raising energy from the earth or other sources to achieve your means.