When you’re studying, as with when you’re at the gym, it’s sensible to pace yourself. Don’t demoralise yourself by taking on more than you can handle; instead, go slow and steady, with regular breaks, so that you can cope for the long haul.
Except that’s really boring, isn’t it?

It’s a bad idea to challenge yourself to ridiculous things at the gym, because you can cause yourself long-term injury. But you’re not going to break your brain because you tried to do something that was a bit too tricky. If you’re bored witless by going over the same notes and the same ideas – but that’s what you have to study, so you’re stuck with it – try to find motivation in making it a challenge.

You might see how quickly you can write a two-page essay, or whether you can solve a Maths problem in three-quarters of the time you’re supposed to.