I find the failure mode for this to have been pretty insurmountable when you’re not genuinely on board with said identity. A trivial example: the behavior you could make into your identity is the result of socionormative expectations and pressure. Doctors say one should exercise, all my friends are athletic and expect me to be, society values fitness, to espouse the exercise lifestyle is seen as a moral virtue, etc.—but what if someone personally doesn’t give a fvck about exercise beyond these external forces? For some people it will never naturally become their identity, even if they “want” it to be, on account of the exogenous pressure to fit this norm. For some it will feel contrived or even forced. It is possible for the inherent motivation to act in accordance with your identity, and how you choose to orient yourself in the world, to be at odds with what you “want” to accomplish, do, or identify with on the surface. For that person, they have to genuinely value being the person who exercises for the “need” to become implicit.

For me this manifested on the intellectual plane. At times in the past I wanted to intellectually be “into XYZ”, like mathematics, for example, or some other subject matter. I had the aptitude and certainly the capacity to focus, enough to develop genuine expertise over time. But then I found that, in truth, I did not genuinely enjoy the subject matter enough to delve so deeply. I liked the idea of being someone who had cultivated rigorous mathematical expertise, but not actually being that person in reality, so it could never be my identity naturally, it would remain maximum effort as long as that were true.

It’s strange to me how we can convince ourselves to want something we don’t actually value intrinsically. But I suspect that’s a longstanding feedback loop in evolutionary psychology.

On the converse side of the story, I posit you can also get pretty far in stagnation of actual personal growth / progress by simply convincing everyone around you deceptively of a specific identity, and have that be validating enough to preclude cultivating that identity in earnest. Another trivial example: the “smartest guy in the room” effect. If you are sufficiently smart, or surround yourself with individuals who won’t challenge you intellectually and default to being deferential, you can certainly feel pretty smart, and have that maladaptively/falsely validate your sense of identity as the smartest-in-the-room type of personality.

TL;DR: I think the “make it your identity, then effort towards action comes naturally” is a powerful model for self-motivation, but it has meaningful failure modes—notably, it’s hard to force if you’re not genuinely into it, and sociodynamic feedback loops impacting personal sense of identity can collapse or even pervert the entire approach.