Your post struck a bit of a chord with me - I exhibit very much the same behaviour.
I have, in the past, been diagnosed with a disthymic disorder, and later with ADD. I’m not so sure that either one is the case. I stopped taking meds on my own initiative, and have been managing okay for years. My parents were still concerned though, and at their request I went to see Dr. Gabor Mate (Author of “Scattered Minds”, an ADD book). I was surprised to hear him say that he didn’t think ADD was my problem, due to my career history, but he suggested that my early childhood was a significant factor in the development of my behaviours. I am an adoptee, from about four days after birth. (Are you adopted, by any chance?) Apparently, being an adoptee puts you at risk of developmental problems, since the mere fact that you are adopted means that your biological mother was stressed during your development. Add to that the fact that I did not have any emotional connection to anyone immediately following my birth, and it sets the stage for the behavioral patterns. I do not like being touched - never did, and my mother (adoptive) recalls me squirming away from her as an infant.
This is one behaviour commonly associated with autism, which someone else already mentioned. In addition, I had a psychological profiling performed which lent some insight into my thinking, but identified an extremely low sentience (zero value, according to that particular test). Sentience, as I understand it, is the characteristic defined by taking pleasure in sense stimula - interestingly enough, this directly contradicts another one of my behaviours, which is to constantly seek stimulation. (I skydive, rock climb, SCUBA dive, work for a search and recovery team that entails swiftwater swimming, used to do search & rescue for the coast guard, etc). This sort of adrenaline addiction is consistent with ADD, which I presume led to that diagnosis.
I was a gifted child, and am still pretty quick on the problem solving end of things (I now work for a consulting engineering firm); however, I am still socially inept, and do not have much empathy for anyone. I also do not experience emotional reactions to events that would otherwise be expected to cause them, as you posted. In my mind, at least, it just makes sense… Why worry about something if you can do nothing to influence the outcome? When you can influence the outcome, deal with it and move on…
The doctors would have you believe that this is all some great deficiency or brain malfunction, will probably have a few dozen names for it, and probably have drugs to sell you to “cure” it. Did I mention I have no faith whatsoever in doctors?..
I think that you and I are uniquely blessed with the ability to exercise rational thought under duress (or, what most persons would consider to be duress). Shit happens. Why get worked up over anything?
-FK
PS - I have to ask why you stayed with your girlfriend after she confessed?