noun or participle which takes 'suru'
Describes dwelling on small worries or regrets, often without taking action. Typically used with する.
小さな失敗をいつまでもくよくよするな。
Don't keep fretting over a small mistake.
彼は試験の結果をくよくよ考えている。
He is brooding over the exam results.
adverb, adverb taking the 'to' particle
worriedly; constantly fretting; obsessing over little things
Adverbial use, often with と, describing the manner of worrying or brooding.
彼女はくよくよと悩んでばかりいる。
She is always worrying about every little thing.