There is a common kind of impatience, felt by ordinary men in the world, which befalls them on account of the inordinate love they have for themselves and for temporal things, which they love apart from God; so that to have them they do not mind losing their soul, and putting it into the hands of the devils. This is beyond help, unless a man recognizes himself, how he has wronged God, and cuts down that tree of Pride with the sword of true humility, which produces charity in the soul. For there is a tree of Love, whose pith is patience and goodwill toward one's neighbour. For, just as impatience shows more clearly than any other sin that the soul is deprived of God--because it is at once evident that since the pith is there, the tree of Pride must be there--so patience shows better and more perfectly than any other virtue, that God is in the soul by grace. Patience, I say, deep within the tree of Love, that for love of its Creator disdains the world, and loves insults whencesoever they come.
St. Catherine of Siena, Letter to Monna Agnese Malavolti, in St. Catherine of Siena as Seen in Her Letters. The whole letter is well worth reading. Earlier in the letter Catherine says that anger and impatience more than any other sins give one a foretaste of hell; which is quite sobering, particularly since, although rarely angry, I am often very impatient. But I can entirely see what she means. And later in the letter she discusses a more specific kind of patience, in which people "imperiously demand from God that He should give them consolations and tribulations in their own way, and not in His."
And yes, she really does mean that one is only genuinely patient if one's response to insults is love.