Thats a hard one because all of my schooling time I was depressed and couldn't concentrate. I wished at an earlier age i found meds that worked for me and gave me the ability to concentrate better.
But before that...i would study in short bursts but make that time count. Like study flash cards over and over until i got all of the material correct. Like id go thru the cards setting aside whatever i missed...then go thru those until i got them all..then go thru the whole thing until i got them all... then i would let myself be distracted for like 15 mins...using an alarm so as not to lose track of time...then go back to studying. If i was sad or down id write down in my journal what was bothering me..then right myself positive affirmations to squash out the negative thoughts..giving myself 15 mins for this...then leave those thoughts then focus on whatever task i need to do.
Basically..give yerself...breaks but dont allow them too much time. If yer easily distracted or can't concentrate..go ahead and think those thoughts but only give them but so much time before u go back to work. Then when u do work, make it count and get as much done as u can until u need another break...but then be mindful and dont let the break last too long. Take deep breaths. Call someone who is positive. Read and collect positive memes.
Good luck.