You are not the voice in your head
The observation that cracks rumination open: thoughts arrive; you don't send them.
Try to predict your next thought. Not think it — predict it, a second before it arrives.
You can't. Thoughts show up unannounced, like weather. And yet we spend most of our lives treating each one as a personal statement, signed and endorsed.
Cognitive scientists call the alternative stance decentering: noticing a thought as an event in the mind rather than a fact about the world or a verdict about you. It is the active ingredient in a surprising number of otherwise different therapies.
The practical move is small. Change "I'm going to fail" to "I'm having the thought that I'm going to fail." The content is identical; the relationship to it is not. You've stepped out of the sentence and onto the platform, watching it pass like a train you didn't board.