What does the underlined sentence mean and what is the topic sentence?

"Anxiety, as I experience it, is deeply rooted in anticipation. If I am required to go somewhere that I know will be potentially agitating, then I can fall into an endless sequence of “what ifs” and theoretical scenarios. What is critical here is the word “theoretical.” What do I know for sure about an upcoming event? What value is there in mentally rehearsing multiple scenarios? How much of my anxiety is simply transferring negative past experiences into these scenarios without any real evidence? I have found that if I stick with what I actually know about an impending event (which turns out usually to be much less than one would think), then I can dump all my anxiety-generating scenarios into my “conjecture box.” Conjecture has little weight in my cognition. Conjecture is something requiring additional data and further analysis. I am much less likely to be anxious about something that cannot be solved due to lack of data. To me, in a very real sense, knowledge is power. The more I know about a future situation, the more I can anticipate the agitating factors and prepare myself. The more I know about what I don’t know, the more I can successfully move the future into my conjecture box, and remove some of its anxiety-producing power."

The above is from the book I study. I firstly don't understand what the topic sentence is. Also I don't really get what the underlined sentence implies.

Many thanks as always.