I've been in your position before... Take it from me, if your scores on the GMATPreps (or any other diagnostic exam) for that matter, are not even remotely close to where you need to be, then you need to reschedule your exam. I believe if you do it 7 days before, you only have to pay a USD 50$ change fee; otherwise you lose your whole test fee. Your quant score is pretty good; you really need to work on verbal. Other members have had great success with the Manhattan Gmat
Prep series for verbal (SC, RC, CR). However, the OG 11th ed should give you a strong starting point. Find out where your weaknesses are in verbal.. SC, RC, or CR... then you can focus your efforts more effectively. My advice for improving RC.. read high level publications in your spare time (the Wall St Journal, the Economist
magazine, etc)... and get acclimated with those types of dense passages. CR and SC can be learned through practice, practice, and practice.
You also mentioned you were drained halfway through the verbal... pardon my bluntness, but get used to it... practice more to start building up your mental stamina... we've all had the exact same issues at one point or another during our prep... you'll quickly find out the GMAT is a test of confidence, mental stamina, attention to detail, and some common sense... I have no doubt that if you start to focus on the verbal fundamentals in the OG book, and do more practice exams or sets to build your endurance, you'll start to see significant improvements in your verbal score...
Hope this helps mate... Cheers,