The two get merged constantly, usually in the same sentence, and the merge is understandable — both are interior, both are unflattering, both turn up in the same corner of the internet. But they behave so differently that treating them as one object sends people after the wrong thing for years.
Start from the plainest distinction available. You can quote your inner critic. Right now, in its exact phrasing, probably in somebody's voice. You cannot quote your shadow, because if you could describe it, it would not be in there. One is a broadcast; the other is a blank spot. Any account that treats them as the same phenomenon has already gone wrong.
Four differences that matter
Visibility. The critic is the most audible thing in most people's heads, running commentary through a meeting and a first draft and a text message. The shadow is defined by absence from view. A method built for finding hidden material — watching your disproportionate reactions, reading what you deflect — is wasted on something that announces itself before you have finished the sentence.
It sounds like somebody. Sit with the critic for a minute and it usually resolves into a specific person's cadence: a parent, a teacher who had a phrase, a first manager. It is an internalised voice with a traceable source. The shadow has no voice at all, because it was never spoken; it is the part that went unmentioned.
Whose side each one is on. The critic believes it is helping, and here is the uncomfortable half: sometimes it has been. A voice that keeps you punctual, prepared, and unlikely to embarrass yourself has a track record, which is precisely why it cannot simply be dismissed and why just be kind to yourself bounces off. The shadow is not trying to help or hurt. It is not trying to do anything. It is material with nowhere to go.
What each does under pressure. Pressure makes the critic louder and more articulate — more specific accusations, better evidence, longer sentences. Pressure makes the shadow visible, because the effort of keeping it out of view is the first thing a depleted person stops paying for. If what arrives at hour eleven is a torrent of self-accusation, that is the critic. If what arrives is a version of you that other people can describe and you cannot quite remember being, that is the other thing.
How they are connected
They are not the same, but they are not unrelated either, and the relationship is the useful part.
The critic is generally the mechanism that did the exiling. A child works out which reactions produce a bad outcome, builds an internal enforcement to catch those reactions early, and the enforcement keeps running for thirty years after the household closed. What it is catching is the material now sitting in the shadow. So the content of your self-criticism is a fairly reliable index of what got excluded: a person policed constantly for selfishness has their own needs filed somewhere out of reach, and a person policed for being too much has their volume filed alongside.
Which gives a practical move that most advice on this subject misses. Instead of arguing with the critic — the standard recommendation, and the reason so many people have spent a year composing rebuttals to a voice that is very good at rebuttals — read it as a map. Not is this fair, which is a debate you will lose, but what would I have to be, for this warning to be necessary. The answer names the thing being guarded, and that thing is usually a capacity rather than a fault: appetite, directness, the ability to disappoint somebody and survive it.
What it depends on
Which of the two is actually costing you decides where the effort goes, and they want different treatment. A loud critic is a self-relating problem with a real evidence base behind it, and it responds to the ordinary, unglamorous approaches — noticing the voice, separating the standard from the person, and treating a harsh sentence as a claim to be tested rather than an instruction. Exiled material is a different job entirely, indirect by necessity, and the one described in how to find the material.
If the critic has stopped being a voice and become a settled verdict — if the sentence underneath is I am the problem rather than I did a bad job — then the thing you are dealing with is closer to shame than to criticism, and it behaves differently again. And for the version of the question that involves other people's remarks rather than your own, why the one bad note outlives the nine good ones covers the asymmetry that keeps the whole system supplied with fuel.