While I was not able to think of or find a poem specifically related to starting a new job or a new "work life," I did think of something T.S. Eliot wrote about beginnings and endings. It can be found in a longer work entitled The Four Quartets, in the final quartet entitled "Little Gidding." The entire long poem is about many things, but the final quartet is concerned with the nature of life: that when one thing ends, another begins. While it is spiritual in nature, the principle still applies.
The lines which seem most applicable to one thing ending one thing in order to begin another are found in section five:
What we call the beginning is often the end
And to make and end is to make a beginning.
The end is where we start from. And every phrase
And sentence that is right (where every word is at home,
Taking its place to support the others,
The word neither diffident nor ostentatious,
An easy commerce of the old and the new,
The common word exact without vulgarity,
The formal word precise but not pedantic,
The complete consort dancing together)
Every phrase and every sentence is an end and a beginning,
Every poem an epitaph. And any action
Is a step to the block, to the fire, down the sea's throat
Or to an illegible stone: and that is where we start.
As I said, this poem does not expressly address the idea of starting a new job; however, it certainly reminds us that ends and beginnings are connected. When a new thing begins, something else has necessarily ended, and vice versa. This is a principle of life. When raising a child begins, for example, the state of childlessness has ended. When a child moves out of the home and begins his own life, the state of being a child-raiser ends and something new begins. That is just how it works.
In terms of a job, when one job begins, another has ended, an action which completes a very normal and recognizable cycle in human existence. Or perhaps it is a new job which is beginning, in which case what is ending is a different life (a life with more leisure, less fulfillment, too much free time, lack of personal satisfaction, or whatever the case may be). This is just as true about a new job as it is about life.
Eliot reminds us that any beginning action we take, including starting a new venture, is necessarily a step closer to the ending. Every step forward, even a step in a new direction, is a step toward "the block," "the fire," and to death (the "illegible stone"). Despite that rather foreboding and dour end, that is what every beginning is.
In case you find that depressing, the fact that beginning somehting new is just a step toward an ending, think back to all the other "beginnings" in your life and consider how they have ended are will end. Some things may have been painful, but many others were not. If you had never begun them, where would you be? Even though it will one day end, moving forward in a new venture is a natural part of life in every way.
We shall not cease from exploration
And the end of all our exploring
Will be to arrive where we started
And know the place for the first time.
Through the unknown, unremembered gate
When the last of earth left to discover
Is that which was the beginning;
At the source of the longest river
The voice of the hidden waterfall
And the children in the apple-tree
Not known, because not looked for
But heard, half-heard, in the stillness
Between two waves of the sea.
"Exploring" (beginning new things) brings great rewards, according to Eliot.