We think that if we have enough willpower, we can fight off every temptation that comes our way. And sometimes it works. Here’s the problem, however. Willpower is your best friend when things go well, but often the first friend to check out when you get weary.
When you don’t want to do something, your carnal mind will give you plenty of reasons why you don’t have to. Your emotions will even join in and say, “I agree, because I don’t feel like doing it anyway.” Your mind, will, and emotions would love to run your life, but the Bible says you’re to be led by God’s Spirit. “Walk and live [habitually] in the [Holy] Spirit [responsive to and controlled and guided by the Spirit]; then you will certainly not gratify the cravings and desires of the flesh” (Gal 5:16 AMP).
Willpower and discipline are necessary to a successful life, but willpower alone won’t be enough. Determination may get you started and keep you going for a while, but it’s never enough to bring you across the finish line. “‘Not by might nor by power, but by My Spirit,’ says the Lord of hosts” (Zec 4:6 NKJV).
What happens when instead of turning first to willpower, you turn to God? He releases His power into your willpower and energizes it to bring you across the finish line. That way willpower doesn’t get the credit for your success, God does. That’s why Jesus said, “Without Me you can do nothing.” This is one of the most important and difficult lessons you must learn if you want to enjoy the life Jesus died to give you.
http://www.jentezenfranklin.org/the-weakness-of-willpower/