Rhetorical questions that I may try to answer myself:
Why do I do more than one thing at a time? Why do I find it so hard to just sit and be quiet with my own thoughts?
If I do not do a number of things at a time (e.g. listen to music, read the web and knit or play WoW and write or watch TV, knit and chat), then one of my thought processes (my brain feels like a multi-threaded operating system, to me, but that will only make sense to other tech nerds) will get a chance to dwell, perseverate, obsess and depress. In the past I have managed to spiral myself down into a bottomless pit of despair and depression way too easily if I thought about "stuff" too much and so I am always doing something.
The only way I can fall asleep at night is to play solitaire, read or do something similar on my iPhone (or, in the past, a PDA). If, instead, I lie there with nothing to distract my mind I end up going round and round in circles in my head about anything or everything that is on my mind (which means I am unable to fall asleep however exhausted I may be) and I have developed this distraction mechanism to let myself unwind and get to a sleepy enough state that, when I put down my iPhone/PDA and get comfortable, I fall asleep almost immediately, 9 times out of 10.
This is also the case at all other times. If I merely knit, for example, without something to occupy my other processing threads (I seem to have at least three) then I will end up thinking about something that is on my mind - or everything that is on my mind so that I end up in a panicky confusion - and I have, at times, sent myself catatonic by doing that. The only times I seem to be able to dwell on anything meaningful is if I am in a safe space (for example, counselling) or if I am writing (because another processing thread is thinking about phrasing, spelling and so on.
I expect that this multi-threading is common with many people but it did ring a bell for me when I was reading about the different types of 'multiple personality' where, instead of having many discrete personalities that take it in turns to be in control or "up front", there are many facets of a person's personality all running in parallel. I doubt that is what I have but it rang bells.
I do know that many different therapists, counsellors and doctors have told me that my mind works really quickly and goes off at tangents a lot. I know this annoys some people in my daily life but I have always been like that, since I can remember first having reasoning powers and self-awareness as a child. This may be why I cannot concentrate on one thing very well (although I did used to be able to do that more when I was younger by letting part of me think about something else). Trying to force myself to single-process / single-task over the years has caused a lot of confusion and distress for me, with teachers, family (and eventually the tapes in my head that we all have of things people have said in the past) and so on all saying that I am a daydreamer and never get things done/finished. Because I was being forced to concentrate on A, my mind would end up trying to do that but get derailed to thinking about B, C, D and E instead.
Maybe that is what people these days call ADiD (inattentive, not hyperactive). I am not physically hyperactive but I do think that my mind is and I get bored doing only one thing (although when I was a programmer I could not multitask at all if one of the tasks was coding, possibly because coding takes more than one 'process' at a time as it is). So, maybe what I am describing is ADmHD (mental Hyperactivity). I don't know.
All I know is that it feels like my eyes need to be doing something (reading, people-watching or watching TV) while my mind does something else (thinking about a conversation) and my mouth does something (talking or eating) and my hands do something (typing, playing a computer game or knitting). All of these seem to be threads or processes that almost hurt to be idle. It is hard to explain.
ETA: This is just like my brain. While reading a few entries on my LJFriends' page I am knitting and remember that I wanted to add about how I can knit without looking at the knitting using some looking-inwards and feeling what I am doing process at the same time as a counting process that is counting rows and/or stitches at the same time as I am reading/talking/watching TV. Maybe I am just an alien. I always used to say I was from Alpha Centauri Four.
ETA2, 20141213: This actually makes a lot of sense if I am an Aspie (have Aspergers).