Epistemic status: I have a good track record for keeping myself motivated, but this wasn't always the case. These are the insights that I've had over the years while I was figuring this out. My beliefs haven't changed much lately and I find myself referring to them all the time.
As always with this kind of murky folk psychology, YMMV, but don't discard these ideas too quickly.
All motivation ultimately stems from belief. If you don't know why you want something, it stems from an alief that is hidden.
The grand unified theory of motivation: Your gas pedal is always 100% pushed. If you don't go forward, it means your subagents are pushing you in opposite directions. Integrate your conflicting b/aliefs and you'll start moving forward. Integrate your b/aliefs completely and you will have solved motivation.
If you believe something is the right thing to do, but you can't bring yourself to do it, then not all of you believes it.
As with people, your s1 will not listen to you if you don't listen to it. The best way to make it feel heard is to pass it's ITT. If you can risk it, endorse the alief publicly. Then you can change it.
Aliefs will be 10x less reasonable if you shut them out. If you embrace them, only the most nuanced part of it remains.
Your s1 won't stop complaining about that marshmallow as long as it's in front of you. Eat it or throw it away, but don't let it sit there slowly deconstructing your internal consistency.
Fatigue can be physical, but it is nearly always emotional, arising from internal strife.
"Ego depletion" only makes sense if you don't self-identify with your s1. If you do, you will simply experience it as a preference
People are just a bag of subagents. If you interact with them, your bags just mix. If they offend you, that's often because they've expressed a subagent that you repress in yourself. Your reaction is how you would react to this part of you internally.
100% of your "will power" comes from your incentive landscape and 0% comes from training. If you want more willpower, make it impossible to do the bad thing. Put your distractions in a time-locked box.
Most of your hidden motivations will be hidden because they're not socially desirable, and/or because they're not consistent with your identity. I am not a paragon of virtue. I am just scared to own up to my vices, and it's keeping me from integrating myself. The same is true for you and for everyone that isn't completely enlightened
You are not smarter than your s1. The truth is somewhere in between. Be humble, listen to it, find a middle ground, become integrated.