How practical is it for you to re-attempt the GMAT in case it doesn't go well? If it is not too much, I'd say don't postpone your current GMAT date. In the worst case you can re-attempt it, however, having a set date can really push you to get more done in less time. If you postpone it, there's no guarantee you will study at the same rate and might again end up in a similar situation again (with more mocks spent).
So my suggestion will be to keep the date fixed and try to figure out how you can improve in this one month. You can start by looking at your weak areas - and then trying to practice more of those. Make use of GMAT club as much as you can for every little doubt you might have. While I won't endorse any test preps if you feel like you may try one of those as well.
Good luck to you and don't give up, you can definitely improve a lot in one month!