It's always been hard for me to put my ideas out there for others to critique. I remember in high school not wanting to read the comments on my papers from my teachers because I couldn't handle the feedback. Even when it was helpful feedback, I had a difficult time. As I've gone through my education and had to put more and more of my content out there for critique though, it's gotten easier. I've learned to read comments several times before I react and make changes. I've had to learn to not take feedback personally, which isn't easy. Getting feedback on an article I've written or paper I've submitted means that I am opening myself to growth.
One thing to think about when preparing for this, is that that the Diagnostic Evaluation acts as a sort of preview to the types of questions or direction for the Comprehensive Exam. For me, this meant questions pertaining to physiological responses to common aquatic stressors, why reciprocal transplant experiments are used, and plant biology. The plants questions are mostly because my score for plants on the Diagnostic Exam was not stellar. Though, in my defense, the only plant courses I've taken have been identification courses! Word to the wise to those wanting to pursue a PhD in Biology, make sure your undergraduate degree is sufficiently broad AND pay attention to those classes!