We suffer more in imagination than in reality, Seneca writes, and Stoic practice on fear largely follows from that. Most dread concerns the future, which is to say it concerns something that does not exist yet and may never.
Their counter-move is not positive thinking but the opposite: rehearse the thing you are afraid of, in detail, until you have taken its measure. The exercise was called premeditation of evils, and the point is that anticipated misfortune arrives smaller than the version fear had been advertising.
42 passages
"The ceasing of action, impulse, judgement is a pause and a kind of death, not any evil."
"Keep before your eyes day by day death and exile, and everything that seems terrible, but most of all death; and then you will never have any abject thought, nor will you yearn for anything beyond measure."
"There are some things which are unable to hurt us, and whose power is exclusively beneficial and salutary, as, for example, the immortal gods, who neither wish nor are able to do harm: for their temperament is naturally gentle and tranquil, and no more likely to wrong others than to wrong themselves."
"Let not the future trouble you; for you will come to it, if come you must, bearing with you the same reason which you are using now to meet the present."
"You have escaped from all accidents, jealousies, diseases: you have escaped from prison: the gods have not thought you worthy of ill-fortune, but have thought that fortune no longer deserved to have any power over you": but when any one shrinks back in the hour of death and looks longingly at life, we must lay hands upon him."
"When a raven croaks inauspiciously, let not the external impression carry you away, but straightway draw a distinction in your own mind, and say, "None of these portents are for me, but either for my paltry body, or my paltry estate, or my paltry opinion, or my children, or my wife."
"With everything which entertains you, is useful, or of which you are fond, remember to say to yourself, beginning with the very least things, "What is its nature?" If you are fond of a jug, say, "I am fond of a jug"; for when it is broken you will not be disturbed."
"You need not think, because sometimes they call for death, that their lives are long: their folly torments them with vague passions which lead them into the very things of which they are afraid: they often, therefore, wish for death because they live in fear."
"I will set sail unless anything happens to prevent me, I shall be praetor, if nothing hinders me, my financial operations will succeed, unless anything goes wrong with them."
"For him whose sole good is what is in due season, who counts it all one to render according to right reason more acts or fewer, and to whom it is no matter whether he beholds the world a longer or a shorter time—for him even death has lost its terrors."
"'Unlucky am I, because this has befallen me.' Nay rather: 'Lucky am I, because, though this befell me, I continue free from sorrow, neither crushed by the present, nor fearing what is to come.' For such an event might have befallen any man, but not every man would have continued in it free from sorrow."
"Now if there is no hurt to the elements themselves in their ceaseless changing each into other, why should a man apprehend anxiously the change and dissolution of them all?"
"If then, when you arrive at last at your final exit, resigning all else, you honour your governing self alone and the divine element within you, if what you dread is not that some day you will cease to live, but rather never to begin at all to live with Nature, you will be a man worthy of the Universe that gave you birth, and will cease to be a stranger in your own country, surprised by what is coming to pass every day, as at something you did not look to see, and absorbed in this thing or in that."
"How admirable is the soul which is ready and resolved, if it must this moment be released from the body, to be either extinguished or scattered or to persist."
"In the same way, also, he who gives way to sorrow or anger or fear, wishes that something had not been or were not now, or should not be heareafter, of what is appointed by that which ordains all things; and that is law, laying down for every man what falls to his lot."
"This you do not understand, and you bear a countenance which does not befit your condition, like many men who sit in the circus or the theatre without having learned that their home is already in mourning: but I, looking forward from a lofty standpoint, can see what storms are either threatening you, and will burst in torrents upon you somewhat later, or are close upon you and on the point of sweeping away all that you possess."
"For one bitten by true doctrines even the briefest and most familiar saying is reminder enough to dispel sorrow and fear, for instance: 'leaves, the wind scatters some on the face of the ground; like unto them are the children of men.' Yes, 'leaves' too are your children, and 'leaves' those whose voices shout and applaud convincingly or on the contrary curse you or blame and rail beneath their breath; 'leaves' too even those who will receive and hand on your fame hereafter."
"Now either the mind of the Whole has an impulse to each individual; and if that is so, welcome what it initiates; or else it had an impulse once for all and what follows is consequential upon that; and why are you anxious?"
"Where work can be accomplished according to the reason which is common to gods and men, there is nothing to fear; for where it is possible to obtain benefit by action which moves on an easy path and according to your constitution, there is no injury to suspect."
"Does he consider banishment to be an empty name, and say, "What evil is there in changing one country for another?" and yet, if permitted, does he not grow old in his native land? does he declare that there is no difference between a longer and a shorter time, and yet, if he be not prevented, lengthen out his life and flourish in a green old age?" His answer is, that these things ought to be despised, not that he should not possess them, but that he should not possess them with fear and trembling: he does not drive them away from him, but when they leave him he follows after them unconcernedly."
"'For really and truly, men of Athens, the matter stands like this: wherever a man takes post, believing it to be the best, or is posted by his captain, there he ought, as I think, to remain and abide the risk, taking into account nothing, whether death or anything else, in comparison with dishonour.'"
"That man prays: 'How may I know that woman'; do you pray: 'How may I not desire to know her.' Another prays: 'How may I get rid of him'; do you pray: 'How may I not want to be rid of him.' Another: 'How may I not lose my little child'; do you pray: 'How may I not be afraid to lose him.' Turn your prayers round in this way generally and see what is the result."
"As he presses forward and makes his way towards the life that is sublime, well-ordered, without fear, proceeding in a regular and harmonious course, tranquil, beneficent, made for the good of mankind useful both to itself and to others, he will neither long nor weep for anything that is grovelling."
"Since I have begun to make my definitions without a too strict adherence to the letter, a man may be called "happy" who, thanks to reason, has ceased either to hope or to fear: but rocks also feel neither fear nor sadness, nor do cattle, yet no one would call those things happy which cannot comprehend what happiness is."
"Do not, therefore, bring to the diviner desire or aversion, and do not approach him with trembling, but having first made up your mind that every issue is indifferent and nothing to you, but that, whatever it may be, it will be possible for you to turn it to good use, and that no one will prevent this."
This is one of the smaller topics on the site, and that is a fact about the corpus rather than an oversight. The Stoics rarely treat fear on its own; it turns up inside their discussions of death, poverty, exile and disgrace, because for them fear is always fear of some particular thing wrongly rated as an evil.
So if the passages here feel thin, the argument you want is probably next door - under death, adversity, or wealth and fame.