Something that has worked very well for me is mindfulness. In summary it is about paying attention to your thoughts and emotions. In the beginning for me it was about paying attention to obsessive thoughts and emotions and noticing when they happened and then doing something like asking myself, "who are you talking to", or, "is that happening now".
It gave me a chance to create a pause between the emotion/thought and the reaction. I now had choices, I could choose how I wanted to respond rather than just reacting.
I now had the ability to stop and not immediately jump on that hamster wheel in my head, or if I was on it, and noticed it, I could choose to jump off.
It is big part of what is taught in Al-Anon though they don't call it mindfulness. It is also a big part of Buddhism and Taoism.
I highly recommend it and you can get lots of information for free on the web if you Google mindfulness.
Your friend,