The way we make decisions is pretty interesting. Making decisions that are bad for us is easy and effortless. Think about how easy it is to decide to watch TV, eat some junk food, take a nap, and then play some video games. On the other hand, let's say that today you wanted to have a really positive day. To actually decide and convince yourself to prepare and eat healthy food, avoid watching any TV, power through your work even when you're feeling tired, and avoid wasting time on facebook is hard. Not impossible, of course, but just by thinking through these two scenarios, you can imagine how much more mentally taxing the latter is.
The trick to overcoming this is to make decisions once that will have an effect for a long period of time-- in other words, having a standard routine that allows for no variance. For example, I want to have a good sleep schedule. I can do what I tried to do for about 30 years, which is will myself to make a good decision on when to go to bed every night, which didn't work at all, or just say that computer is off at midnight no matter what, and I'm asleep by two no matter what. Now I don't get to make a decision every night-- I just turn of the computer, read, and go to sleep. All I had to do was make this decision once, and then train myself on it for a couple weeks before it became second nature.