The best time to reflect on what you’ve accomplished is before you get to work on it in the first place.
Don’t get me wrong – I believe in reflection, even if I’m not great at it. But if you’re trying to specifically reflect as a learning exercise, then it’s not worth very much unless you’re comparing it to a prediction. Remember the scientific method? First you form a hypothesis, then you experiment. If you experiment first, you’re exposing yourself to all sorts of bias.
So if you’ve just accomplished something and you want to reflect on how you did – don’t. You don’t have a counterfactual, you’ll attribute to skill what might be luck, and you won’t know how to repeat it yet. Instead, form a hypothesis about how you might do it again – and how you might repeatable-ize it. Then when you’re done, you’ll have something to compare it to, and your reflection will be worth far more.