Seems more like a bad day than any fundamental problem with your preparation.

You have all the right materials (a little too much of it, actually) and you've approached it well.

GMAT Prep generally provides a good estimation of the target score. But at the end of the day, that's what it is - an estimate.

There's plenty of time for Round 1 application deadlines. If you have the inclination, you might want to spend some time brushing up the concepts and re-taking the test.