I just took the GMAT(again) and did much better the second time around...I too had a similar problem, I would always be so conscious about if I encounter an "easy" problem that I tend to panic because I feel like I got the last question wrong. During practice tests, all I can say is stay focused on the task at hand and use the techniques that you have used in class. This worked great for me especially on verbal(helped me focus on reading comp looking for scope of passage and tone and sentence correction looking for the vampires errors). Also instead of practice tests, try to do 35-40 problems in a row to build your mental(and emotional) stamina and see how many you get right. Hope this helps.