I wouldn't feel that bad if you've only been studying for a week, unless you're scheduled to take the test in the immediate future. Although I didn't have much of a quant background before I started studying for the GMAT, I'm a really fast learner, but it still has taken me quite some time to understand some of the concepts. And the difficulty level of different GMAT resources varies. For instance, the other night I did problems from Manhattan Review Turbocharge, and the question seemed way different and more difficult than the
OG.