心烦意乱 describes a state where you're too upset to think straight — your heart is vexed and your mind is in chaos. It's commonly used in writing and speech to express emotional turmoil.
grammar
This is a parallel compound idiom (并列式成语): 心烦 (heart vexed) + 意乱 (mind chaotic). Both halves express the same idea with different words, a classic structure in Chinese four-character idioms.