I hate to say it, but in the absence of spontaneous motivation, discipline works wonders.
I once sat in on a masterclass with the great concert violinist Isaac Stern, and someone asked him how he stayed motivated when he practiced violin. He replied that every day, he would choose one small thing, and he would simply work until it was perfect.
It's a good basic technique. Choosing discrete tasks -- like say, reading 10 pages of your Chemistry assignment and doing the problem set -- and chopping your work up into manageable bites can make it seem much less taxing than having a huge big pile that you have to somehow get through. Pick one thing that you are going to do, then do it. Then give yourself a little break. Then pick another thing and do that.
Often, I find that the motivation comes once you start doing something -- it interests you and the next thing you know, you really don't mind so much.
Start a Revolution -- Stop Hating Your Body!