finding a normal at a point

dear all,

please look at the following figure. in this, i am trying to find normal at the corners and d sides of this shape. i used the 'polyfit' function, to generate the slope, and then find the line.

at the corners, the normal is somewhat okay, but at the middle point of the sides, the normal is not proper. the reason i attribute to : a very small or large value of inverse of 'first coefficient' given by polyfit.

suggest a way out...