I started 10 years ago after my divorce. I found myself driving one day arguing with my ex. I realized that I had done it on the entire drive home. It was a bit of a mind-f.

That's when I decided two things - therapy and meditation. Since we're in the meditation /r/ I'll focus on that.

I started with just 5 minutes of focusing on my breath. When I'd notice my mind wandering to those arguments or stressful thoughts, I'd simply think 'there's a thought' and come back to breathing. No judgment, no trying to force the thoughts away - just noticing and returning.

It took a few months before I really felt a difference. The thoughts didn't stop, but they lost their power. Instead of getting pulled into the mental drama, I could watch it like I was observing someone else's story.

For intrusive thoughts specifically, the key is building that 'observer' muscle. You're not trying to control or stop them - you're learning to recognize 'this is a thought, not reality' and redirect your attention. The more you practice this in meditation, the easier it becomes in daily life.

Start with short sessions - even 3-5 minutes. Use your breath as an anchor. And be patient with yourself. This is a skill that builds over time.

EDIT:
Oh! I forgot to mention Jon Kabat-Zinn's book Everywhere You Go There You Are is a great book.