I personally find that what is harder than summing up the energy and motivation to do school work, is to actually figure out what to do.
It's very vague (my experience with prelim) - just "go do some work"
E.G. I have a maths assessment in 13 days, so I went to my (2) textbooks and I put sticky notes on every excercise (20 of them) that were related. I then went to something like microsoft excel, drew out all the days i had, and put them evenly over the days (ensuring i get as much exposure to each topic per day, rather than a lot of one another day).
The thing about plans is, when its sitting right infront of you - "tuesday: 7-12", you just do it, you dont sitttt aroundddd trying to have a think and ponder about it. It is of course harder for other subjects, but you get the point.
Once you know what to study and its right infront of you in my opinion you should find it far more cruisy
If that helps :\