After seeing several posts today that reflected this, I thought that maybe people didn't really need any motivation on Monday. The motivation, encouragement, and reminders seem to really be important over the weekend.
I think the bottom line is that you need to keep your goals in mind all the time.
I'm not saying that you need to obsess or freak out over what you're doing all the time, but you do need to be vigilant. It's not supposed to be a punishment. It may take some getting used to, but that's all.
Staying disciplined takes practice. It takes time and effort. Some people say it gets easier, some people say it doesn't ... the truth is, it doesn't matter. If you want to lose weight, then you have to make the effort to do so. Don't think of it as a sacrifice. Think of it as doing something incredibly good and positive for yourself.