This may not be especially helpful at the 3-week mark, but it can be good motivation to look back on your progress periodically. If body composition is part of your goal, take pictures periodically, and do "before and now" comparisons (you don't have to show anyone else; this is strictly for your own benefit). Do something that used to test your limits and notice that it feels easy now. Keep a log of little aches and pains, and review it to see how they've quietly fixed themselves.
Fitness can feel Sisyphean at times. Zooming out a little not only lets you feel more accomplished, it can get you excited for the future.
Unfortunately, I'm not sure this technique generalizes well to non-fitness endeavors.
There's nothing quite like feeling you've made no progress only to look at a previous progress photo and barely recognize the person as you.
"Cryonics has a 95% chance of failure, by my estimation; it would be downright /embarrassing/ to die on the day before real immortality is discovered. Thus, I want to improve my general health and longevity."
That thought has gotten me through three weeks of gradually increasing exercise and diet improvement (I'm eating an apple right now) - but my enthusiasm is starting to flag. So I'm looking for new thoughts that will help me keep going, and keep improving. A few possibilities that I've thought of:
Pride: "If I'm so smart, then I should be able to do /better/ than those other people who don't even know about Bayesian updates, let alone the existence of akrasia..."
Sloth: "If I stop now, it's going to be /so much/ harder and more painful to start up again, instead of just keeping on keeping on..."
Desire: "I already like hiking and camping - if I keep this up, I'll be able to carry enough weight to finally take that long trip I've occasionally considered..."
Curiosity: "I'm as geeky a nerd as you can find. I wonder how far I can hack my own body?"
Pride again: "I already keep a hiker's first-aid kit in my pocket, and make other preparations for events that happen rarely. How stupid do I have to be not to put at least that much effort into making my everyday life easier?"
Does anyone have any experience in such self-motivation? Does this set of mental tricks seem like a sufficiently viable approach? Are there any other approaches that seem worth a shot?