Thinking about NOT doing something keeps that something at the top of your mind and makes it more likely that you will do it. If you put a bottle of booze in the middle of the kitchen table and remind yourself NOT to drink every minute of the day then the odds are that you will wind up drinking that bottle of booze. You have set yourself up to fight the "Beast Voice" that tells you to drink every minute of the day--eventually you will get tired and give in to the "Beast Voice".
On the other hand if you decide to have a day of abstinence from alcohol and you plan for a busy day full of fun activities to do instead of drinking then you will be likely to succeed. This is because you will seldom think of drinking when you are doing these fun things. On the few occasions when the "Beast Voice" does pop up and tell you that you need a drink it will be relatively easy to defeat it.
Herein lies the contradiction of will power. Will power is a great aid when you use it to concentrate on attaining a positive and it backfires when you use it to attain a negative.
If you use willpower to tell yourself not to drink then the act of drinking is constantly in your mind and you have to constantly fight the "Beast Voice" that is telling you to drink. If you use your will power to do fun things instead of drinking then the thought of drinking seldom appears in your mind and you spend much less time and effort battling it.
Will power is your friend if you use it wisely.