Do you have a set process for CR and SC? Like with CR, you start with the question, analyze the argument, re-read the question, eliminate wrong answer choices? The more you have a set process that you can follow successfully the less you end up wondering if you are going wrong.
Mentally, you have to trust yourself and the process. Learn how to speak the language of the GMAT. What I mean is when you work through practice CR and SC problems, go back and review them thoroughly - both the questions you got right and wrong. If you get something wrong, don't read through the explanation and say "ok - I got it". Immediately go back to the problem and see if you can get it correct for yourself. If you can, read just enough of the explanation to give you a clue and go back and work through the problem. As you slowly and methodically work yourself through lots of problems like this you start to see how the GMAT "thinks".
Once you understand how the GMAT works on SC/CR, just trust yourself and stay true to your process and you'll be ok.
KW