If you want to build a work ethic, start small.
Personally, I've constantly struggled with having my work done at the last minute. Like you, it has stuck with me all my life, up to when I reached secondary 4, which is around 16 years old.
Maybe it is my country's environment which forced me into this mold I've never wanted to attempt to fit into, but hey, it's for the best right? (Hopefully) Perhaps in the long run, I can produce consistent and promising results and feel proud for something I've achieved through my hard work, which is based off my work ethic.
Like anyone else, start from somewhere. You don't have to be an upbeat lad and go, "Alright I will do x amount of homework and make sure I clear off my overdue work and make sure I score straight As for my major examinations!", oh please don't do that, you would just screw yourself over.
Start small. Try clearing overdue homework piece by piece, perhaps 3 pieces of homework in one day. You say you have difficulty concentrating, I say you are just easily distracted by the electronic devices all around you (I am guilty of that). Maybe you have an illness which causes you to have difficulty concentrating, perhaps that might be the case too.
Enough of diverging from the main point. If you were to start the habit of clearing your homework day by day, and incorporating some extra homework which you took the initiative to work on, your work ethic will slowly begin to form. It may not be the best, but you can slowly refine it to be consistent and it might even be something you can take pride in.
Sorry for my long-winded reply, this felt way too relatable. I hope it helps. All the best.