The phenomenon of "compassion fatigue" has been studied extensively. Everyone has their limit of giving before reaching this point. It creeps up on you till one day you realize it. The only solution that works for me is time off, even if for just a day or two. You've been busting your butt studying and working for so long it is almost inevitable you'd feel this way. It's not a state of not caring; it's a matter of exhausting your stores of what you have to give. Those stores can be replenished, but it takes time, IME.