on intrusive thoughts

I sat down to meditate last night, and an unpleasant thought popped into my mind. I squeezed my eyes shut and used all my mental energy to stop the thought - it was a visual thought that I didn't want to see. I promptly identified it as an intrusive thought, as I have identified many other "intrusive" thoughts over the years. Then I wondered what makes this thought intrusive, but other pop-up thoughts not intrusive, thus beginning a think session before meditation proper. I don't really know how my thoughts bubble up, even the ones I'm looking for, but when I have desirable thoughts, its because my focus is pointed toward them. My attention is on them. And it is there purposefully, because I want my attention on them. When I'm meditating or just going through my day, thoughts will pop into my head. A lot of replaying social situations, anticipating social situations, worrying about the future. Stuff like that. When those thoughts pop up, I'm often caught up in them, they pull me, I…