If you feel that you were really affected by not practicing with problems on the computer, then I would recommend getting Kaplan Premier. I don't believe the book is the best on the market for teaching the material, but they have a lot of questions on their cd that would allow you to study and practice problems on the computer.
In terms of what you can do differently, I would recommend going through each Manhattan Guide again, except this time around, make sure you constantly review the material that you complete early. For example, lets say you finish the Number Properties book and move on to Fractions Decimals and Percents, make sure you go through the Number Properties book twice a week. Otherwise, you will forget what you learned. Do this all the way until your exam and you will do better. good luck
