Why the loop gets shorter

Look at where the last box points. It does not point back to the trigger. It points back to the behaviour.

That is why this speeds up over time. At the start there is a trigger, and the trigger is something recognisable, being tired or alone or bored or having had an argument. Later on the loop stops needing one, because the shame from the last time is now sufficient on its own.

People experience that acceleration as proof that they are getting worse as a person. It is not. It is a loop that has found a shortcut.