This forum has tons of valuable information already populated within it, to help you with where to start. Take a look below and see if there are any good bits of information on resources you haven't already encountered.
Also, how was your diagnostic? were you above 50 questions right? If so, practice problems, practice problems, practice problems, and work on your method of solving the exam - many people go through and do multiple passes with easiest / quickest questions first, saving the more complex and unknown for the end.
Sorry to hear you have to retake, but hopefully this next time will be the one!