Don't be discouraged by the mental stamina -- that's something you have to build up to and good chunk of the battle! However, if you feel like you're wasting your time, it might be a good idea to reassess your study habits. I agree that a schedule might help, but drilling your weak spots might help as well (e.g. if you struggle on MSS questions, drilling several of them in less time than the 'target' may help you get faster). Good luck!

I agree it probably isn't feasible to finish everything, but it sounds like you're working smart by prioritizing where you need to study! Making sure you have the core concepts down is key.