For the monastics, it was crucially important to remember the divine providence in all of life's trials and tribulations. It is no different for Isaac the Syrian. He writes:
Provided a person remembers that it is through the will of God that all times of respite and of vexation take place for a person -- when God bids it, he has respite, and through His will he is allowed to be afflicted by whatever it may be -- however much he has to struggle, he will be continuously without anger or vexation in his mind all his life, through all the changes that press upon him, through providence, in body or in mind. With this knowledge his heart will exult in serenity, and he will gaze in great peace, in hope, towards God, giving thanks for His providence towards humanity -- a providence which He dispenses continually in accordance with what is fitting for them (Isaac the Syrian, Second Part 25, 1).
For Isaac, peace of mind in times of trouble and tribulation is possible because we remember that all things happen to us by God's will. If times are hard, that is because God is permitting that certain things afflict us; and if times are good, then it is thanks to God's grace and his favor in blessing us.
Of course, the implicit premise in all of this is God's immutable goodness and love. That is how it is possible to remain at peace in times of trouble and vexation: because God loves you, and is in control of everything that happens to you; therefore he allows these things to happen because it is fitting, for your own sake.
Isaac goes on to say:
Such a person does not blame the demons in his vexation, or his fellow human beings, or the body -- for it is from these three immediate sources that all vexations that exist derive -- since he is aware that it is God who sets these things into motion: it is not through the will of any of these three that his respite or vexation takes place, but rather it is through the will of Him who sets them into motion.
When Isaac says that these things do not trouble us through their own will, I take it that he means: they do not act independently of God's providence, as if God had control over them and even opposed them powerlessly. Rather, God sets these things into motion, and everything that happens to us is a part of God's plan. There are no events which are outside the reach of God's providence or care. Isaac would insist, furthermore, that it is only in these way that we can keep from despairing. Otherwise, if some things are outside of God's control, then his goodness and power -- the only grounds of our hope in the world -- are compromised.
God makes use of us, too, in his providence. Notice this wonderful line Isaac has at the end of this section:
For not a single restful thought is put into motion in us except as a gift that comes from God. And down to the smallest kindness that someone does to us, that person is moved to do this by God, and this action is needed for the setting aright of our lives.
The small good thing that someone did to you which brightened your day was a gift from God, as was the comforting thought which gave you peace in a time of trouble. Indeed, if this post is of help to any person, it is a gift of God to them through me. We have God to thank for everything that comes our way, for the good which is enjoyable and for the bad which is yet ultimately for our good. Because we know that the Lord works everything for our good, and nothing lies outside of his control, we can retain peace of mind and tranquility even in difficult times.