(...) doing something for the very first time. You’re stressed out because of the fragile material and you make some mistakes but mistakes are just about you're placement on both sides of this line you draw. When you cut a miror it may happen a break, a line you don’t put enough strengh into and the angle of a triangle falls apart. "You were too much on your left, maybe too much on your right. Take your tool to draw the line firmly at once."
You don’t make this kind of mistake twice because you know when the glass is bending or breaking (...) and you want to look like a very smart guy understanding the deep construction of glass even if you absolutly don't get it. You have to find the balance to play with material to be more comfortable with it. That's it. Maybe you think too much and don't feel comfy ; leave your overworking brain in your locker.