Why is it so hard to change?

Why do I keep doing the very things that I don't want to do anymore? Why is it so hard to change? I am motivated, I know that I need to change and I know what to do but something keeps me from doing it. The devil will use anything to keep you from doing what you know is right and from being happy. If you don't feel good about yourself for whatever reason the devil will use anything to keep you from doing what you KNOW you need to do and he will do it at all costs. The devil wants you to be miserable.