Hi Matt,
I'm really sorry to hear that you didn't get the score you were looking for. I know it can be frustrating, but I'm glad to hear that you're determined to stick with it and achieve your goals!
It's hard to give advice without specifics. For that reason, I highly recommend that you reach out to your Kaplan teacher in person. He or she will be able to look at your practice test performance, talk about how you did in classes, and hopefully give you some personalized recommendations that will help. But, I can take a shot in the dark over the internet: whenever I see a student whose real test is much lower than their diagnostics, I always ask them about the day before and the day off the actual test. Did you sleep? Eat? Study too much, stay up too late? Were you nervous or stressed out? If you can pin down any factor that might have negatively impacted your score, you are that much closer to eliminating the problem and doing better on your next go around.
Hope this helps!
Eli