How do you discipline your child

As a mom and an employee, it's really hard sometimes to really have time for kids especially for yourself! For me it's a little harder since my work is really far from home, I leave my house when my kids are still asleep and go home when they had already finished their dinner. Well I guess I'm still lucky since I still arrive home and spend some time to play with them, but how do I discipline them? I have a big problem with my eldest, we've been going to a therapist bec. he's always short tempered, we would receive some comments from the school saying that he is bullying his classmates but for him it's just his lambing time, he can't determine if it's hurts his classmates feeling, like collecting all the crumbs of the eraser and throwing it to his classmates... it's really frustrating, at home he don't want to share his toys and quickly irritated when his younger brother get his toys, I've been really traying to like asking him to go get a quiet time, but sometimes he don't want to listen, and now that we are preparing him for big school I'm really worried! Some doctors say it's the terrible 2 - 5 kids years... but I just hope that it's true...