i dont think theres anything i can tell you that you dont already know..
but anyway, i'll try..
o.k in order to do something, you must understand what it actually is and what it involves.. i.e look up concentration in the dictionary (it may sound like a silly excercise, but there have been books written on this very thing)
Im sure some of us have to deal with people several times a day who are busy doing many things at once, including listening to you or whatever the case and im sure we've at some point experienced the consequences of this action, usually resulting in a mess up or misunderstanding of whatever was arranged or discussed.. ( im not saying multi-tasking is recipe for disaster, im just saying it takes practice and its not too wise gambling on your capacity to do this when you cant afford to have to deal with the actions of it possibly not working out )
I dont want to stray to far off, but what im saying is, the very idea of concentration is to withdraw from other activities so as to deal with particular ones more effectively.. Its about focusing your mind on relevant things and excluding your focus from others... a working example would be,
take note of the exam you have tomorrow, but dont
focus on it.. Theres nothing you can do about that right now, so just put your focus in the only place where your concentration will be of any use.. this could be studying before the exam, or even while writing one, the idea still remains, concentrate and focus on whats at hand at that moment, its the only area your efforts can produce results at that particular time..
Dont allow yourself to be distracted for too long.. its much harder to get into the swing of things when you allow yourself to stray.. Taking a break is important yes, this can be a few minutes during the exam, while studying etc. but do still try and maintain a certain amount of focus on the task at hand, dont just bomb out completely, like i already said, its harder to get back into it if you've gone too far into lax mode.. (i hope this makes sense)
its really just about striking a balance between, work, interest, concentration and focus..
If you ever look into any of these things, you'll see, its all very controllable..
Some of what you say seems more related to fatigue ( im assuming so since you seemingly do have healthy academic results ), its pretty simple; fatigue is overcome with good regular sleeping patterns, balanced diet and so forth, just dont over do it on whatever energy supplements you may take, this can be very distracting and the same applies for any calming substances too.. I dont generally recommend any, but its up to you in the end whether or not you feel you can benefit from them..
i hope i have helped and that all this makes sense, i didnt include as much as i could have, but im sure you can source far more detailed and richer information around the web, in your library etc.
Hopefully the sparse information ive provided will help you at least a little bit..
once again, best of luck..

EDIT: just saw this thread
here specfically on this topic, it doesnt seem to be heading to anything constructive, so im not going to post there.. however you may take a look at it if you like..