Hi, Marianne. You asked an interesting question.
"Thoughts" maybe often more like brief intrusive thoughts or images. These are indeed sometimes very difficult to "challenge." There are several cognitive behavioural techniques that may apply. First, rather than challenge intrusive thoughts and images, it sometimes helps to simply identify them as "intrusive" or "obsessive" thoughts that don't "mean" anything at all. Intrusive thoughts are common. Most people have them. What makes an important difference is whether or not you attach any importance to them. So, for example, most people experience intrusive aggressive and sexual images. Such intrusive thoughts are "normal". But most people do not attach "meaning" or "importance" to them. They are "just a thought". Its like when you have a dream that doesn't make sense. You can say "It was just a dream and dreams don't make much sense sometimes" or you can start to worry about what it "means" about you. Sometimes our thoughts don't "mean" anything. So , one strategy is to label intrusive thoughts and images as "intrusive" or "obsessive" thoughts and see if that takes some of their power away. "its just an obsessive thought".
Sometimes people have trouble challenging their anxious thoughts because they haven't followed their fear far enough to find the "real deal". So, a second strategy is to "follow the fear" as outlined in the panic program to see where it leads. If you are having intrusive thoughts or images, is there an underlying fear that you haven't identified yet, such as a fear that you will be responsible for something terrible happening to somebody else or some other horrible consequence? Try doing a thought record "on paper" and really following your fear to see where it leads. Challenging thoughts on paper and really "following the fear" to discover all of your underlying fears and assumptions isn't easy but is often worth trying, even if you have been doing it on your own and "in your head" for a long time.
Finally, a third CBT strategy to challenge intrusive thoughts and images is to expose yourself to them. So, for example, one thing you can do is make a tape in which you describe the intrusive thought or image in as much detail as you can and "follow the fear" by describing what happen