Is Half-Hearted Better than Not at All?

Maybe this 30-day challenge thing was a bad idea. I'm struggling a lot with actually motivating myself to do it every night. I seem to have a lot of trouble following through on challenges, so I don't know why I'm surprised at this. I was doing a 30-day workout challenge, and that actually lasted about a week, similar to this challenge. I've done a soda free challenge, where I try to avoid drinking pop. That didn't last too long. I've tried it multiple times, and honestly I average about a day on that. I feel like I'm a trustworthy, dependable person, so why can't I succeed in these challenges, especially when they're dedicated to my health and well-being?