to alternate between joy and worry; to be swayed back and forth by ups and downs (often used to criticize overreacting to short-term changes).
株価の小さな上下に一喜一憂してはいけない。
You shouldn't be tossed between joy and worry over small fluctuations in stock prices.
試験の点数に一喜一憂するより、次の勉強に集中した方がいい。
Rather than being happy or upset about exam scores, it's better to concentrate on the next study.
チームは結果に一喜一憂せず、長期的な目標を見据えている。
The team doesn't swing between elation and despair over results; it keeps its long-term goals in mind.