"He suffers more than necessary, who suffers before it is necessary." - Seneca
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This is my single favorite quote from Stoicism. Related to Michel de Montaigne's quote in Weekly I/O #28.3, "A man who fears suffering is already suffering from what he fears."
It's almost cliché to say we should not worry about things we can't change. That said, I think the ability to worry less is more like a skill we have to practice than an inborn personality that not everyone can have. If we think, "I just cannot be less worried about those things", we would always suffer from unnecessary worries.
Likewise, Stoicism (and its Eastern friend Buddhism) is something that requires practice. Of course, it is hard or sometimes even seems impossible to be free from fear (like I'm still sometimes irrationally afraid of taking an airplane, encountering bees), but it is definitely worthwhile to practice avoiding to be suffered from it consciously.