I don't know if it is one intrusive thought, but I can see that I am easily and constantly focusing on something intensely. It doesn't have to be on something that makes me anxious, it doesn't have to be on DPDR itself, it can be anything. I am trying to figure out something, I am interested in something, I am upset about a conversation I had, I am thinking about how to solve some interesting problem at work... anything. And I am so focused I am kind of absorbed in my thoughts. And if I am absorbed by something, it means that my surroundings exist less and less to me.
And it's like when you listen to loud music in your car, part of your attention is on the music, and as you lower the volume, part of your attention naturally goes back to the road, you start to hear the sound of the engine again, be more present to your surroundings, and so on. But the more you increase the volume of the music, the less you can pay attention to what is around you (kind of), as if the volume of the rest was lowered. And for me DPDR sometimes feels like my thoughts have a very loud volume and take all my attention, like I am very preoccupied with something and the volume of the surroundings is lowered, like it exists less in my mind, it has much less weight. But the problem is that when I stop focusing on my thoughts and go back to my surroundings it feels like their volume doesn't come back up, it's like it is stuck to a low volume. Even when I try to "look outside of my mind", my mind is still in self-absorbtion mode.
But my mind is very strong when it is trying to focus intensely on something, and it feels like it is my only strategy for every problem. So when I see that the volume of my surroundings is too low, I see that this or that doesn't feel real anymore, and I naturally try to focus on something to try to make it real again. But as I am doing that the volume of all the other things decreases again and I am left with that thing only, like with a tunnel vision, just like what I do with my thoughts. Focusing is what makes my surroundings disappear and gives me that kind of tunnel vision, and yet when it is disappearing I try to use even more focusing to solve the problem, which doesn't work.
If I try to "stop my thoughts", I will focus on them, or focus on stopping the thoughts, and it becomes yet another absorbing task. And if it is absorbing, it means I will tune out from the rest of reality again.
What I am trying to do now is to not use that focusing strength to solve problems but try to relax and just do nothing. Not to "focus" again on my surroundings to try to make them come back, or focus on a symptom, or focus on a thought, or intellectualize about what it actually means to relax and focus on that meaning, or to try to focus on a new technique that will help me stop focusing, or focus on my surroundings and try to bring their volume louder by sheer mental force, or I don't know what. Just to try to relax a bit more than what I do usually. Because relaxing doesn't come by just focusing on a different object, or by focusing on relaxation. (It reminds me of Charly Chaplin in "Modern Times" who works in a factory screwing nuts on bolts all day, and when he stops he is stuck and can't help trying to screw everything because he is stuck in that mode, and just like that I feel I am stuck in "focusing mode"
But still, I don't know if it can help. And everybody is different. It sounds a bit like meditation, but still, my several past years of meditation practice, sometimes quite intense, did bring me many things and many cool sensations but it never really decreased my actual DR and emotionnal numbing significantly or on the long term. So this is maybe, most probably, not a definitive answer. But knowing myself I know it can't hurt me to relax on the focusing and have more energy to pay attention to other things.