Wow. I can completely relate. I have been pulling my hair out more lately, and I specifically do the same thing. I especially like pulling the ones that are shorter mainly because I know that they are short because I pulled them out before. My thing that I used to do was pulling out strands that felt "right." Those ones just felt better when pulled, as in no pain. Honestly, I still do that. I am a little embarrassed about my hair pulling and my bf always gets upset and tells me to stop when I start pulling out my hair in front of him...

edit: Just to clarify, I don't know if I have trich myself, but I do have the same type of compulsion that you described.