You wouldn't say it to a friend
Big mistakes, old voices and other hard cases
If you've really done something bad. Sometimes the mistake is more than a late form. Perhaps you betrayed a confidence, or neglected someone who needed you, or made an error at work that harmed people. The rule still applies, and it asks more of you here, because a serious wrong calls for serious correction and repair. You need a full acknowledgement, a real apology, whatever amends are possible, and a hard look at how it happened. A friend who loved you wouldn't tell you it didn't matter. She'd say, "That was wrong, and you know it. Now what are you going to do?" Hating yourself isn't part of what you owe, and it gets in the way of the rest. If the thing can't be repaired, because the person has died or won't speak to you, then the task is to carry the regret honestly and to be different from now on. A counsellor can help with regrets of that weight.
If the voice is someone else's. Some readers will have recognised the commentary at once as a parent's voice, or a teacher's, or a former partner's. If that person was harsh but ordinary, it often helps to say so when you name the remark. "That's Dad." It reminds you that the judgement was never yours and that you're free to disagree with it. If that person was abusive, the commentary may be tied up with painful memories, and changing it may stir them. That's work to do with a therapist and not alone. Your GP can refer you, and in England you can refer yourself to NHS talking therapies.
If you really are bad at something. The commentary sometimes has a point on the facts. You may be poor at spreadsheets, or at keeping in touch with people, or at parking. The friend test doesn't require you to deny it. A friend would say, "Yes, spreadsheets aren't your strength. Do you want to get better at them, or arrange things so it matters less?" A weakness in a skill is a practical matter, and you can address it practically. It says nothing about your worth. The commentary's trick is to slide from "I did this badly" to "I am bad", and you can decline that step while accepting the facts.
If you work in a harsh field. Some professions and some sports run on fierce criticism, and you may feel that being hard on yourself is part of the job. High standards certainly are. But the evidence in chapter 3 came partly from demanding pursuits, and it found that self-criticism went with less progress. The best performers tend to be exacting about the work and steady about themselves. They'll say "That passage was out of tune, do it again slowly." They don't add "you're a fraud".
If someone else criticises you. When criticism comes from outside, the commentary will want to join in on the critic's side. Apply the friend test to the criticism itself. If a friend had received this comment, what would you tell her? You'd probably help her sort out which parts were fair and useful and which said more about the critic. Then take the fair part as correction and act on it.
If the remarks are about your body or your face. The mirror is a favourite location for the commentary. You wouldn't say to a friend what you say to your reflection, and she'd be hurt if she heard you say it to yourself. The rule applies as it stands. If your feelings about your appearance or your eating are taking over your life, that's something to take to a GP.
If compliments are hard to take. As a first step, when someone praises you, say "thank you" and nothing else. You don't have to agree inwardly. Just don't contradict them aloud. It's easier on the other person, and over time it lets some of the praise in.
If you're worried you'll overshoot. Some people fear that they'll swing from self-attack to smugness. I know of no evidence that it happens. Someone who's been hard on herself for decades, and learns to stop, doesn't become arrogant. She ends up about as fair to herself as she's always been to other people.
If it comes with perfectionism. The two often travel together, and there's a separate book in this series on perfectionism. The short version is in chapter 3. High standards help, and the self-punishment that comes with them doesn't.
If your mood is low and the commentary is relentless. Go back to the preface. Relentless self-criticism together with persistent low mood, poor sleep, and loss of interest in things is a reason to see your GP. Therapies that work directly on self-criticism exist and are available on the NHS.