There is no way words can fully capture the experience of prolonged periods of intrusive, obsessive thoughts without the relief of prescribed medication and/or coping mechanisms like CBT.
I (barely) survived that way for about 8 years, starting about age 18. When I wasn’t self-medicating with alcohol, a peculiar but viscious and nearly unbearable form of mental torture would ensue. My brain become my tormentor, hitting me with hour upon hour of repetitive, anxiety-inducing thoughts over trivial matters or extremely unlikely future tragedies.
It was a nightmare I pray never to revisit. And if you are there, I pray for you.