Stopping obsessive thoughts

This problem seems to come on and off for me. It lasts for quite long like 3-4 days at a time. The problem is even if I think to myself "I will definitely not think about xyz", that exact things just keeps coming back. I was trying this technique of neutral observation and simply labeling the thoughts as mental image, talk etc whenever they arise. This technique works better than the former but nevertheless the habit is so strong and the thoughts are really subtle that most of the time, I realize the forming images and mental talk many seconds later.

I was reading about the mindfulness technique - Shinzen Young who follows an algorithmic approach which kind of suites my learning style. There he talks about watching the entrance of the mind like watching the mouse hole. This concept seemed to be highly useful when I read it but its really vague. Where do I watch?. The mind is non physical. Its like the obsessive thoughts just pops up from nowhere again and again. I read that there is a silent undercurrent from which all thoughts originate. Maybe I need to reach that depth of observation to "watch the entrance".

There is another issue. If I really decide to sit in a place and concentrate, no thought pops up. These thoughts pop up usually when I'm busy doing something else, a neutral activity or something which I don't particularly enjoy. Ive also read that this is common, meaning when we really concentrate on watching thoughts they don't come up. They come out only when the mind is in the twilight zone - thoughts called twilight demons. It seems like a tricky thing to learn.