If you won't be able to get the score for psych this year, what makes you think you'll get substantially higher next year? Look at it this way:
Say you get 38 for Psych. Welp, that's not the 40 you wanted - so, we work harder, and get a 41. That's an extra 3 aggregate points. In full honesty, you're not going to get terribly higher if you repeat the unit.
However, what about what would've been your 6th subject? Welp, the moment you get a 30 in that subject, you've already matched that extra 3 aggregate points. Get any higher than 30, and you've already done better than you would have if you had redone psych.
IMHO, there's no point repeating a subject, unless you've scored lower than was required for a given course (eg, want to do commerce, got 24 in methods). For any onlookers, I would also only suggest this if you did said subject in year 11 - there's no point repeating year 12 if you're meant to be graduating that year.