|(Image from blissfulbelle.org)|
Psychologists would say that's thanks to the sixth stage of change: relapse. Change, no matter how big or small, is thought to go through a cycle: pre-contemplation, contemplation, preparation, action, maintenance, and then relapse, which takes you all the way back to the beginning. You can think of the first five as the steps of a ladder, and the sixth being a thunderstorm.
While it's hard (if not impossible) to prevent relapsing, there are steps we can take to try to keep it from happening. For one, being aware of it can help us prepare. On those days where you feel like taking "just one little break" have a plan in place to make it up, or trade it out for something else instead. It's also great to have a new start date. For example, if you know you're going on vacation and won't get that writing or exercise time in, plan to start back two days after returning (that way, if you feel burnt out on the return day, you wont feel bad about taking one more day off, and if you feel up to getting right back into the swing of things, you'll feel great that you're ahead).
Also, know that if you ever do slip up, it doesn't mean you can't get back to where you where. If you made it a first time, you know you can make it a second!
So get back to those new years resolutions that started slipping last month. Try sticking to the goals you set for yourself, or try setting goals in the future.