Passage
Thou shalt not be afraid for the terror by night; nor for the arrow that flieth by day;
Thou shalt not be afraid for the terror by night; nor for the arrow that flieth by day;
Psalms 91:3 Surely he shall deliver thee from the snare of the fowler, and from the noisome pestilence.
Psalms 91:4 He shall cover thee with his feathers, and under his wings shalt thou trust: his truth shall be thy shield and buckler.
Psalms 91:5 Thou shalt not be afraid for the terror by night; nor for the arrow that flieth by day;
Psalms 91:6 Nor for the pestilence that walketh in darkness; nor for the destruction that wasteth at noonday.
Psalms 91:7 A thousand shall fall at thy side, and ten thousand at thy right hand; but it shall not come nigh thee.
The verse centers on "thou", "shalt", "afraid", "terror", "night", "arrow", and "flieth". It is saying that the verse draws attention to "thou" and "shalt", so its meaning should be read from those terms before moving to application.
The nearby context moves from verse 4's "He shall cover thee with his feathers..." into verse 6's "Nor for the pestilence that walketh in...", so "thou" and "shalt" belong inside that flow. In Psalms context, the local focus is worship, trust, the LORD's kingship, and covenant mercy.
A plain takeaway is to answer the verse's own emphasis on "thou" and "shalt" with trust shaped by these words, not by a vague optimism outside the passage.