To envy someone is to look away from your own value derived from who you are as you are. So in this sense there is an ignorance of the value you inherently posses when envying someone. To imitate I perceive to mean it is a death to who you are. Pretending to be someone else who you may admire or envy results in a death of authentic self. Hence this may be what is meant by imitation is suicide. Therefore there is wisdom in "... that he must take himself for better, for worse, as his portion."
When he refers to "no kernel of nourishing corn can come to him but through his toil bestowed on that plot of ground which is given to him to till." I believe the plot of ground he is referring to our own personal mind and body. This is what each of us has been given to till. To prepare and work and plant within it the unique seeds which can only grow in our own unique mind and body. When we are true to ourselves and take action in life from this place of authenticity we will find true nourishment. But to seek what others have or seek to be what others are then we wont find true nourishment and who we are as we are will go unappreciate as we look outside of ourselves for fulfillment.
"Ne te quæsiveris extra." or "Do not look for yourself outside." This leading line is said over and over throughout his essay in a miriad of different explanations and is at the heart of the verse mentioned in the question.
