I 100% agree with you...the mindset you need for NCLEX-style questions is so different from other tests. All of the answers can be correct, but which is MOST correct? Ughhh. When I first started answering NCLEX-style questions in classes, I was terrible. I think it just comes with practice. I usually go through and read the question first and picture that situation in my head. "What would I do here?" Then I look at the answers and hopefully my answer is in there. If not, I go through each one and ask myself why it is or is not correct.
Sorry, I'm still terrible at the "select all that apply" questions, so I have no advice for you there
Does your school use ATI for practice tests? I'd use those as much as possible. At least how my school has it set up, you are able to see the rationale immediately after you submit each question.
I'd do the NCLEX questions at the end of each chapter and if your textbook has a free site (usually with an activation code), use that...they have a lot of study resources and NCLEX questions on there.
And yes, buy a NCLEX book