Of things some are in our power, and others are not. In our power are opinion, movement towards a thing, desire, aversion, turning from a thing; and in a word, whatever are our acts. Not in our power are the body, property, reputation, offices, and in a word, whatever are not our own acts. And the things in our power are by nature free, not subject to restraint or hindrance; but the things not in our power are weak, slavish, subject to restraint, in the power of others. Remember then, that if you think the things which are by nature slavish to be free, and the things which are in the power of others to be your own, you will be hindered, you will lament, you will be disturbed, you will blame both gods and men; but if you think that only which is your own to be your own, and if you think that what is another's, as it really is, belongs to another, no man will ever compel you, no man will hinder you, you will never blame any man, you will accuse no man, you will do nothing involuntarily, no man will harm you, you will have no enemy, for you will not suffer any harm.
The Enchiridion of Epictetus (trans. George Long)
To be happy in life, according to Epictetus, one must first learn to discern between what is in one's power and what is not in one's power. Over what does one have control and over what has one no control?
Obviously, a man will be unhappy and disturbed if he works constantly to control what is beyond his power to control. Likewise, a man's will and self-control will atrophy if he neglects those things over which he may exercise his power.
I find many unhappy Christians simply because they believe they have control over things that are beyond their control. "You cannot change the color of a single hair on your head," said our Lord Jesus Christ. We have far less power than we believe. When sickness or death invades their life, they are upset because they feel powerless; they should not be surprised, for what man has the power to control death? It is only that now their self-deception is unmasked and they realize what they should have known all along: this is one (of many) spheres in which they have no control.
On the other hand, there are things over which we do have power, yet are convinced that they are not in our control. Chief among these is our will to do good for our neighbor. When faced with some opportunity to be charitable, many a Christian (myself included) mistakenly says something like, "I desire to do some good here, but it is not in my power." Perhaps fears concerning the future assail us or questions such as, "If I give this person aid, I may not have enough to feed myself tomorrow." Here we allow things outside our control to worry us out of exercising control over what has been given into our hands.
Perhaps our Lord Jesus had reason to demand an immediate and thoughtless charity in His Sermon on the Mount; self-fulfillment is to be found nowhere if not in the exercise of our will where it may be exercised - here and now - and upon those things which are in our power.
How miserable we are and what victims we claim to be when we believe ourselves to have control over what is not in our control, for when the truth becomes clear, we are bereft of every comfort our illusions gave us. Likewise what victims we become when we believe ourselves to be powerless even in those few matters that belong to us, and how full of regret we must be doomed to become once we have wisdom to recognize those times control was had and yet we did nothing.