If you're looking for reassurance that every aspect of your life will be incredibly balanced, you won't find it here. Work and school are going to be the only priorities in your life and your social life will revolve around them. If your social circle gets smaller, you basically just weeded out the people who weren't going to be with you through thick and thin because everyone else will understand that these last two years are crucial.
The lack of sleep is something that varies from person to person. I volunteered two places, worked, and had 9 courses last year, and I still managed to get 6 hours a night on average. Sleep is crucial.
You're going to have to learn strict discipline and time management, which will pay off when you finally get into your graduate program.