I am a PhD student, 4 years into a very difficult program (ecological/dynamical systems psychology... basically mathematical/physical psychology). My attention span has increased significantly from when I was an undergrad. Why? Discipline and hard work. I'm not implying you don't work hard. What I mean is that you need to re-train your brain, though. The first two years of my program involved courses where I has to read on average 500-800 pages per week (in addition to research and teaching obligations) in order to prepare for a weekly oral exam that was administered in front of my peers (it was very embarrassing to be unprepared for that!). This is what forced me to concentrate better. It wasn't fun, it sucked real bad, but I'm much. much better off for it.
Now, it does take me about an hour to get focused enough to do work, but once I get there, I'm good. So, my only suggestion is that you have to keep trying, keep reading, for many hours, even if you think you're not getting anything out of it, You are, it just takes time to see the results (kind of like working out). There might bet better way to go about this, this is just how it happened for me.