You made a mistake. You are not a mistake. Keep the verb and the noun in separate drawers.

There is a wide difference between something you did and something you are, and shame collapses the two in about a second. Once the error becomes a description of you, nothing can be fixed, because you cannot repair a fact about your nature. Keeping them apart is not letting yourself off the hook. It is what makes repair possible at all.

When the collapse starts, say the specific thing you did in plain words, with no adjectives about yourself attached. Then name the specific repair. Regret about an action moves you somewhere; a verdict about your character simply sits.