You can try these:

Analyse your recent tests immediately, if you haven't. Find out where you go wrong and then discuss your mistakes with an expert. Devote the next 2 days on this exercise and then take your last test.

Remember, there's little knowledge you can gain in the next few days. What you can gain, though, is some confidence by:

(i) Going through all the concepts that you've studied. Go through all the questions that you've marked as important ones.
(ii) I know it sounds like a cliche, but 'Believe in the hard-work you've put so far'. You've scored more than your expected score in atleast a couple of tests. That should give you some confidence.

General pointers (for the day of the GMAT):
- Read the problem/question very carefully. Most mistakes happen here.
- Don't take a problem/question on your ego. If you've spent enough time on a question, and are still stuck on it, Move Ahead!
- Do not study one day before your GMAT. Studying just before the exam might do more harm than good.

If you can follow these basic pointers, you will surely get a better score than what you've been getting so far in your mock tests.

All the best!