Number properties encompasses some of the most difficult questions on the GMAT, both in PS and in DS. This is normal for all candidates. I referred to all my practice materials (OG 11 & 12, Princeton Review and Veritas Prep workbooks) to go through as many problems as I could and at the end, I got somewhat more confident than I was at the beginning. A lot of practice also helped me to understand some of the key concepts of number properties which are tested again and again.
By the way, I have read in this forum that Manhattan Review Maths Guide is a pretty good resource; you may try that along with the other materials you already have.
