Shame
Shame is the engine underneath a great deal of this, and it is the emotion least likely to be named out loud — because naming it requires describing the thing you are ashamed of. That silence is most of what keeps it running.

Guilt and shame are not the same emotion
They feel similar and they do opposite things. This distinction is the single most useful thing on this page.
I did something bad.
I am something bad.
Points at an action, which can be repaired.
Points at your existence, which cannot be repaired, which is why it produces paralysis rather than movement.
Produces apology, change, and then it ends.
Produces hiding, withdrawal and self-punishment, none of which end anything.
Proportionate to what happened.
Frequently enormous, and often triggered by something small or by nothing identifiable at all.
Survivable in company. You can tell someone about guilt.
Specifically resists disclosure. Telling someone is the thing it exists to prevent, and it is also the only thing that reliably reduces it.
Ends when the repair is made.
Is not reduced by repair, which is why over-apologising keeps going long after forgiveness has been offered.
The cycle, and where it can be interrupted
Four stages. Each is easier to interrupt than the one after it, and almost everyone tries at the last.
What actually reduces it
Almost nothing works on shame internally. You cannot reason with it, because it is not a proposition — it is a stance towards yourself, and arguments against it get absorbed as further evidence of how much explaining you require.
What does work is disclosure to one person who does not recoil. This is a well-supported finding and it is also the hardest possible instruction, because avoiding exactly this is what shame is for. One person, one thing, told badly, is enough. It does not have to be a therapist and it does not have to be the worst thing.
The second thing that works is the guilt-shame distinction applied deliberately. When the wave arrives, ask what specifically was done. Not who you are — what you did. Almost always the answer is smaller than the feeling, and it is repairable, and repairing it is available in a way that being a different person is not.
The third is timing. Shame peaks and falls like any other emotion — usually within a day. Almost every self-punishing decision is made at the peak, and almost none of them survive a night's sleep. Deciding in advance that nothing gets decided during a shame wave removes most of the damage without requiring you to feel any better.
In the book, chapter 4 is the shame trap and chapter 15 is what you do about it. Chapter 13 on validation comes before both, and the ordering is not accidental — self-compassion offered to a reaction you consider illegitimate does not land, which is why 15 without 13 tends to fail.
Shame is the only emotion that reliably weakens when it is described to someone who does not recoil.