Eby's theory is that many of us have circuits that try to prevent us from doing the things we want to do.

I don't think the anthropomorphic frame is helpful, though. It's like saying that a thermostat is "trying" to prevent the sun from warming up the room as it "wants" to.

It'd be a bit more accurate to simply say that we can have control circuits that require mutually incompatible states or actions.

It seems that evolution combined an explicit goal-driven propositional system with a dumb pattern recognition algorithm for identifying the pattern of "pursuing an unreachable goal".

PCT has a more general form of this idea that doesn't require a specialized notion of an unreachable goal: it proposes a reorganization system that responds to chronic or intrinsic error signals by reorganizing the control circuits involved until the error goes away. An unreachable goal would just be one example of what could trigger this system.

PCT considers this "reorganziation" system to be the basis of trial-and-error learning as well, that is, continued failure to reach a goal prompts a series of variations in behavior until the error signal goes away. In the case of an unreachable goal, the reorganization will simply continue until the organism "gives up" - that is, reorganizes the control system so that the current condition is no longer considered an error, and thus experiences no motivation to pursue it.

This isn't to say that we don't have any ability to handle unreachable goals specially, just that the PCT model doesn't need one.

These long-term plans do not immediately return a reward signal, so they trip the "unreachable goal" sensor inside most people's heads, causing them to completely lose motivation.

This isn't true. I can get immediate reward signals thinking about my goals over the next year or two. The real issues have more to do with whether your thoughts also trigger any error signals due to predictions of difficulty, stress, having to give up other things, etc. When those things are present, any positive reward is going to get drowned out by shorter-term negatives.

One might ask why on earth evolution designed the power-balance parameter to vary with your mood;

It didn't, because the model you're describing here is unnecessarily complicated. Ego depletion is simply a function of conflict between controllers -- PCT predicts that two systems in conflict will trigger maximal activation of the neural pathways involved in the conflict, like two competing thermostats simultaneously running the heat and A/C at maximum. And this would naturally expected to result in overuse of brain fuel (e.g. glucose).

Conversely, if someone does badly and then gets criticism for that bad performance, their "unreachable goal" sensor will trip out and remove their will to continue, creating a downward spiral of ever diminishing motivation.

This isn't really how it works, either. The difference between success and struggling is in one's interpretation of events, not the events themselves. A successful person responds to negative events with, "Ah! I love a challenge!", or at worst, "Well, I guess I learned one more way that doesn't work."

Conversely, a natural struggler's interpretation of good things is that they won't last, aren't "real", or "don't count".

Dweck's research into mindsets also shows that it's ridiculously easy to get people to think in this mindset, even without exposing them to any actual adversity whatsoever! So, it's not a matter of exposure to adveristy or success; a person who has nothing but success in their early years may spend the rest of their life handicapped by it. (The downward spiral of many child stars being an all-too-obvious example.)

But we should definitely think about developing much more effective ways to defeat that enemy within

You can't defeat it, and you don't need to. What you need is to resolve your actual priorities. I'm still integrating the results of PCT into my own work, but the effects have been mindblowing at times.

This weekend, for example, I realized that every time I ever tried to "define my priorities" in the past (as requested by virtually every self-help book in existence), I was actually trying to make a list of what I thought my priorities should be... rather than what they truly were. And just realizing that tiny difference in perspective has completely changed my outlook on planning, and what I'll be doing in the next few weeks.

The point? While I was trying to do what I thought I "should", I was essentially ensuring that I would always be in conflict, always fighting this so-called "enemy" within. But by instead doing what I really want in the first place...

There's no longer any "enemy" to "fight".