I have been in a situation where I have had the entire day completely free, and the only thing I wanted to accomplish was to get to the gym and somehow never ended up getting there. And I have also been in a situation where I had a terribly busy month and didn't skip a single workout.

The answer to why this happens is all because of the habit loop. I highly recommend reading The Power of Habit.

Here's the loop: Cue -> Routine -> Reward.

Basically, if you become aware of your habit loop and make the necessary tweaks, you will be able to accomplish literally anything.

So pick anything you want (that's the routine). Now if you're not struggling with it, great. But if you find yourself to not be as committed and as consistent as you would like to be, start tweaking the cue, the reward, or both until you no longer struggle.

The following are usually the cues:
Location
Time
Emotional state
Other people
Immediately preceding action

The reward can be anything, it's very subjective and personal.

Okay, now let me give you examples of how this has really worked in my life. I had so much free time a few months ago, and I told myself I'll go to the gym whenever during the day, I have nothing else to do. The result: I ended up being really inconsistent with my training. The routine was working out, the reward for me is how I feel about myself while working out and afterwards, but the cue was completely missing. There was no cue. The way I have my workouts set up now is, when my girlfriend gets home from work, we change and go to the gym. To someone who does not understand the habit loop there is literally no difference between this and my "I have all day, I'll go to the gym whenever I want." However, look at the difference in the cue. I went from no cue to:
Time: About 6PM.
Other people: My girlfriend.
Immediately preceding action: She just got home from work.

I have found that I am pretty consistent this way. Obviously if that wasn't enough, I could tweak it even more. Add more cues or add stronger rewards.

I'll give another quick example. I usually go tan 15 minutes every day and my schedule is very flexible. I don't have a schedule. So I thought the same thing with tanning as well. I'll just go whenever, I have all day. I ended up being very inconsistent. So all I did was tweak the cue. I decided I was going to go right after I woke up, and I haven't missed a day in about a month now.

So figure out what you want to do which is your routine, and then tweak the cue or the reward until you get to the level of consistency that you desire. It's really simple and you can literally do anything with the right kind of changes to the habit loop.

Let me know what you guys think.

Cheers.