Why do some people do it and other can't. Motivation for the better result is more powerful than the temptation for the excuse. We are talking about Personal Power, within ourselves, which dictate to us what to do, why to do it, when to do it, and how to do it without a question. Instantaneously those all get answered like a plane on auto-pilot.
The goal, this permanent change in the way you live, is at Your discretion at every moment of every day. Like the flicking of a lightbulb switch to get your result of light. You tell yourself how it is going to be to get that result. If you lose track for a moment, just switch the light back on. Your powerful side of you must switch you mind over to what you want.
Don't feel powerful enough to make your mind up for the right choice. Keep trying. I like to think that if your not getting the results you want and motivation is the issue you either need a bigger carrot or a bigger stick. Most need the bigger stick, like "this will kill you if you don't...", or something else serious that makes it of the utmost importance like "I'm going to lose my house if I don't....". Keep the carrot and the stick metaphor in your head constantly to get those results, it works. Got to get that carrot, and I definitely don't want to feel that stick hitting me. Most situations can can be dramatically improved by setting your mind up with these powerful tools to inspire the correct direction.
Try it seriously for yourself and see. If you don't succeed get your mind wrapped around thoughts that motivate.