To think deeply about something to the point of worrying or brooding; to torment oneself with anxious or distressing thoughts (same as 思いつめる).
彼女は仕事のことで思い詰めて、夜も眠れないという。
She is so worried about work that she says she can't sleep at night.
そんなに思い詰めないで、一度気分転換したらどうですか。
Don't brood over it so much — why not take a break and change your mood?
失敗を思い詰めるあまり、体調を崩してしまった。
She worried so much about the failure that her health suffered.
子供が学校のことで思いつめた顔をしていた。
The child had a worried look on his face about school.