I've never had problems concentrating but my girlfriend was diagnosed with ADD when she was younger, but never took any drugs for it because her mom didn't believe in it. She got very bad grades all through school, because she said she just couldn't make herself pay attention or care or do homework. She barely passed high school, and really only did because her math teacher liked her and realized she was going to be a musician.
In the first semester of college, she likewise almost failed out, and was placed on academic probation. Then the second semester she met me and started smoking most every day. Instantly, she found she was able to apply herself to school, and ended up graduating with honors. Now she's in graduate school and has yet to get anything but an A... on anything. I'd call that quite a turnaound
But, I still wish she'd try to get some Adderall... good stuff
Now, keep in mind I don't really believe in ADD either. It was just a matter of her seeing herself and her situation from a different perspective and realizing she had to take responsibility for her actions, and not blame them on some phantom mind disease.