Eknath Easwaran's Commentary
It is not so much work that tires us, but ego-driven work. When we are selfishly involved, we cannot help worrying, we cannot help getting overly concerned about our success or failure. The preoccupation with results makes us tense, and our anxiety exhausts us.
The Gita is essentially a call to action. But it is a call to selfless action, that is, action without any selfish attachments to the results. It asks us to do our best, yet never allow ourselves to become involved in whether things work out the way we want.
It takes practice to learn this skill, but once you have it, as Gandhi says, you will never lose your nerve. The sense of inadequacy goes, and the question “Am I equal to this job?” will not arise. It is enough that the job needs to be done and that you are doing your best to get it done.