A little stress is good, but no to the point where you feel like your productivity is being reduced.
Don't risk burning out... there's still a while to go.
+1. now is the time to start improving. This includes clarifying stuff you are not sure about, revising stuff you are sure about, etc.
It's hard to say what marks you should aim to get for next assessment- ultimately rank is the important thing for internals.
However, definitely give it your...
+1.
Invest in your future. If acing umat is important to you, you would'nt mind forking out e.g. $50 for 10 past papers (which I happen to have=)
Practice makes perfect