Write these down only as a trick, as something your mind does, rather than getting drawn into them. Write how your mood changed and how you moved away from the thought later, if that is what happened - how you forgot it, and how the subject in your head turned into something else entirely.
Watch a magician twice and the way you see the trick changes. Do the same with your mind, and give your own name to what it is doing. The first time you see the impossible; the second time you begin to see what may actually be happening. The mind's dark moments work the same way as the sleight-of-hand artist, and their repertoire is surprisingly small. Instead of saying "I'm useless", say "my mind broadcast that agenda again - the one where I'm useless".
The exercise is simple. Write the loud thought down word for word - not the polished version, the actual sentence going round in your head. Then stand in front of the stage and name the trick the magician is doing: it is the same agenda I used to hear; it is the voice of the teacher who was never kind to me; it is so-and-so's voice.
Naming does not kill the thought, and it doesn't need to. It demotes it - from a narrator you believe to a character in a play. The sentence that was the voice of truth keeping you awake becomes an actor in a scene, performing a scene you have watched before. The feeling often stays a while after the naming; that is normal. You are not arguing with the thought, only refusing it the microphone, refusing to let it speak as though it were the supreme commander. And add some positivity and compassion too, in the sentences where you describe what is going on, strengthening your standing as someone who looks at the scene from a positive angle.