In book 8 section 47 of the Meditations, Marcus Aurelius writes,
If thou art pained by any external thing, it is not this thing that disturbs thee, but thy own judgment about it. And it is in thy power to wipe out this judgment now. But if anything in thy own disposition gives thee pain, who hinders thee from correcting thy opinion? And even if thou art pained because thou art not doing some particular thing which seems to thee to be right, why dost thou not rather act than complain?—but some insuperable obstacle is in the way?—Do not be grieved then, for the cause of its not being done depends not on thee.
Things in themselves are not nuisances, rather we make them so ourselves. Nothing is either good or bad in itself, although we commonly think they have to, and incorrectly. The thing is, though, that our thoughts, unlike external events, are within our power, and so we are able to change our thoughts, judgments, and perceptions to make things bearable. Aurelius tells us that if we are annoyed by ourselves, we oughtn’t blame it on others or on anything; instead, we should take to correcting our opinions, as they belong to us, so we can fix them. No one stops us from changing our way of thinking except ourselves. Further, he points out that when we think things we would rather not think, we often complain to others and to ourselves, ignorant of the true nature of the annoyance; as such, he advises to simply change our thinking when it appears to be straying. When we notice negative thinking, acknowledging it and knowing it is the cause of our problems is one thing—but actually acting on it and changing it, is another, and is what we ought to do. But often times we will impute our misfortune to some external thing, such as the day, leading to remarks like, “Today is not a good day,” or, “Today does not like me”; however, if we attribute our personal torment to something impersonal, something external to us, we should be doing the opposite, really, for if “day” is what is causing our problems, we know that it is a force greater than us, outside of us, and therefore not concerned with us. Because the courses of our days are outside of our power, we can do nothing to change them, so instead of resisting them, we should allow them to carry out as they please, as per nature, and leave our attitude to ourselves. Hamlet expresses this in the same way: “[T]here is nothing either good or bad, but thinking makes it so” (II.ii.265-66.). Therefore, our perception is what affects our attitude, so your life is what you make of it.
For further reading: Meditations by Marcus Aurelius (2014)