Reflection is sometimes sold as a “do-it-once-in-a-while-and-you-will-be-fine” thing. Structures in organizations emphasize that as well – quarterly earnings, yearly performance reviews, etc.
The reality is that it works something like this – Have an idea -> Reflect -> Do something -> Reflect -> See outcome -> Reflect -> Tweak your idea -> Reflect -> Try again -> Reflect -> See outcome this time -> Reflect.
As long as you are attempting things, reflection is what ensures you are learning from the process.
Reflection, nearly always, makes you realize you are inadequate. The point of reflection isn’t to feel good. The point of reflection is to learn. And, getting comfortable with feeling inadequate is the first step to learning.
There hasn’t yet been an exercise coach who has found ways to make people fitter without having them experience pain.