It does not sound like attention to me as much as training and interest. I sucked at reading comprehension too and yet read 5+ books a week. The difference? If I don't get to pick what to read (and how) then I ain't paying attention, can jump all over the place, just like you :-)

No one just decides to "concentrate" and does. It's a skill like anything else and has to be built up; little kids have short concentration spans so teachers have to start training them, bringing them back to task over and over as long as it takes, a little at a time. Sounds like you didn't get or "failed" :-) that portion of elementary school, weren't made to do your homework? or did it spottily?

Daydreaming is much more attractive as it takes no "muscle", you just drift along. I got through college and to about age 41 that way! I "work up" in the middle of a final exam where I caught myself wishing I had studied more (so the problem I was working would have been easier to remember how to solve). From then on I made myself do my homework, I made myself go to classes, I made myself do the work/attend the final exam, study, etc. and my grades went from C's to straight A's (this is in my 40's and 50's; I got a single A in typing in 9th grade and an A- in phys ed (badminton) in 11th. But got a second degree with straight A's and went on to grad school (also A's).

If you want to read a particular book; make yourself! If you want to write the script, make yourself; I also recommend you try Script Frenzy: http://www.nanowrimo.org/en/scriptfrenzy (you're in luck, it takes place each April) which will not give you time to daydream. I finally wrote a novel with NaNoWriMo and I'd been trying to do that for 15+ years.