If you mean you are having trouble concentrating, it is a matter of the mind. It is no easy task, but you'd be amazed at what you can accomplish if you work at it. You have the ability to pay attention, it is simply more difficult for you. If you mean, however, physical tiredness, it is probably from the medication regimen. Ritalin may be approved to be safe, but it can do rather chaotic things to one's mind, unfortunately. :u
I can't pay attention unless it's something I want to do, otherwise I have a lot of energy.
Well that is a given for most people. You just have to work at it. It comes easier with time, want, and practice.