Poison

Fitness to a task (or a relationship) - it's there or it's not.  It's not always that another person is a horrible monster. Sometimes it's that you yourself are not fit to abide them.  We're all on this earth for a purpose. We must find our purpose, what we are fit to accomplish, and apply ourselves to the task.
When we take on a task or relationship that exceeds our strengths, the task/relationship can poison our effectiveness, thwart our progress, and leave us damaged, unfit for our God-given task, which then does not get done, or done well.
It is not a failure to acknowledge our limitations, but a triumph, and a necessity.  We may wish we were fit to a particular task, but we abandon our real mission if we allow chasing the diversion to weary us into ineffectiveness.
Don't give in to the tendency to see it as failure to reject the task or relationship in favor of your primary purpose.  See it as having been led to the best path, while rejecting what was never meant for you to begin with, or which now exceeds your ability to  maintain.