I don't know about the PSB, but here are some study tips. Don't take the ATI practice exams right away. Study the material in the ATI book and do the practice problems. IMO, the material in the ATI book is pretty accurate, but the science material isn't detailed enough. The questions on the actual exam can be detailed and pretty much any topic in bio, A&P, chemistry & micro are up for grabs. I thought khanacademy.org and Chemistry for Dummies were pretty good resources. Also, borrow the McGraw Hill TEAS exam book from the library. When you feel you've had a pretty good understanding of the material, take the first MH exam, study your weak areas, take the next exam, etc... Save the ATI exams for last, because those are most like the actual exam, again, with the exception of the science section. The science sections on the MH books are more like the actual exam. If you do well on the MH exams and the ATI practice exams, then you will do well on the actual exam. If you do well on these exams, then you don't really need to purchase the online exams. They are quite a bit harder then the actual exam, and can be quite discouraging.
Don't work yourself over it too much. Take a deep breath; know that you did everything that you have done to prepare yourself; and think through every question carefully. Make sure you understand what the question is asking. Finally, don't panic if you don't know the answer. On the science section, I had absolutely NO idea on the first four questions. Never saw the material in a practice exam and couldn't even narrow down the answer to three choices, let alone two. I thought I knew about 60-70% of that section with confidence, could narrow down about 10-15% to two answers, and not very confident with the rest. I scored 85% on the science, which I was very happy with considering how little confidence I had with that section.