Good question, bigboymaximus - keep in mind that, because the GMAT is a test of "how you think" and not "what you know", it's different from a college exam. A simple mistake can mean that you do 90% of the work correctly and still end up with an incorrect answer, so it may not be that you're a "450 student" (I'd actually argue heavily otherwise given your record of academic success), but rather that you're 90% of the way to being a 600+ student, and just need to focus on that last 10%.

I'd recommend this for your next 10 days:

1) Return to your practice test and determine two categories - mistakes that you made on questions that you should have gotten right; and concepts that you need to learn/relearn that you just didn't know.

2) Progress through the Official Guide keeping an eye out for those mistakes and the ways that the questions are written to twist your thinking, and get a good feel for the style of questioning on the test so that you can recognize it on test day.

3) Find resources (probably online if your test is in 10 days) that will help you learn the concept areas that you didn't quite know or with which you didn't quite feel comfortable so that you can better attack those on test day. Use the Official Guide as a (sorry that this is redundant) guide to which concepts are worth your investment of time. Concepts that appear frequently are worth some of your limited time; you may want to be judicious with that time on concepts that aren't.

It's certainly not impossible to improve from 450 to 600 in 10 days - I've seen it done a number of times - so I'd definitely encourage you to give it a shot. The GMAT is a tough test, though, so don't be discouraged if you don't pull it off...just learn from the experience so that you can build on it when you do have enough time.

Finally, I have to ask about that June 10 "last chance". If you're taking the exam on May 13, you should technically have to wait 30 days to take it again, so you may have to wait until later that week to take it.