GMAT preparation is easier than the other parts of MBA application; b-school life is tougher than application; Out of b-school, live is also stressful.
烦躁 may indicate you may have minor depression (majorities are due to lack of rest and/or excercise). For your own health, quit your MBA dream unless you can handle stresses.
I have the same feeling. Maybe due to working as well, I found I am very easily distracted by everythings. But for this condition, I really don't think I can handle all. So I decide to quit my job and focus on the whole process on my way to chase my dream.