My first ever PowerPrep (old GMATPrep) test was a 540. First real GMAT test was a 620. Increase of 80 points.
My second go around studying, my first GMATPrep was 560. I did hardcore studying for two months and took the real test and got a 700 even. Increase of 140 points.....80 points higher than my first official test. Had I gone through all my materials (only got 1/3 of the way through my books), I am confident I could have reached 730-750.
You first need to focus on what you are getting wrong. Take 10-12 practice CATs. If you can afford a tutor, that may help you focus on your weak areas.
You also might want to give yourself more time. Push the test back.....
BUT IT IS CERTAINLY POSSIBLE!