What if I am thinking stressful thoughts on purpose?

I have once had an obsessive thought about sth. I remembered that I recently read that if you just let it be it will go away on it's own. So I just let it be and it went away. But than I thought "but if I think about it again it will come back".And then I started thinking about it on purpose to prove to myself that there is still danger. It has been two years now and I am still thinking about it on purpose. It is not a particular thought now. It is just a feeling of fear. Then I always get very anxious and can't sleep at night. Sometimes I forget about it for a while. But when I notice that I am not thinking about it it's like I can't help but think about it. I think the problem is also that I am in a constant fight/flight mode because of my physical condition (because of nerve, vene and arterial compression)

I'd love to hear your opinion on this.