How can I convince myself that the grass isn't always greener on the other side?

I always think that the grass must be greener on the other side of the fence. Whatever subject I study, I think about all the other subjects I'm not studying. Whatever work I'm doing, I think about all the other work I'm not getting done. Whatever book I'm reading, I think about all the other books I could be reading. I want to do everything, and I want to do all of it right now. How can I convince myself to be happy with what I'm actually doing and able to do? How can I stop this perpetual cycle of boredom and longing for change?