Old Habits Die Hard

What does that mean, old habits die hard? I figured it meant that some habits are hard to die, right? But I want to know how to form a habit that is hard to die. And who says it only takes three weeks to establish a habit? Why, when I do something like exercise or not eat sugary substances for more than three weeks and come week four, I could take one bite of sugar or skip one day of exercise and it’s all over. My little sister complains about the same thing, she says she could do something for three years, miss one day, and boom, done. Habit over. Doesn’t seem to die hard to me. My little sister and I share that gene.

I really do want to establish a good routine for exercising and staying away from sugary substances. I’ll hear all the usual responses, but I want to know the real trick to make those two things—those habits—to die hard.