1To firmly believe something is true, even without sufficient evidence; to assume something without confirming it.
彼は自分が正しいと思い込んでいて、人の意見を聞かない。
He is convinced that he's right and won't listen to others' opinions.
会議は来週だと思い込んでいたが、実は今日だった。
I was under the impression that the meeting was next week, but it was actually today.
誰も助けてくれないと思い込んで、彼女は一人で悩んでいた。
Convinced that no one would help her, she suffered alone with her worries.
子供のころ、サンタクロースは本当にいると思い込んでいた。
As a child, I was firmly convinced that Santa Claus really existed.
2To firmly resolve or set one's heart on doing something.
彼女は必ず大学に合格すると思い込んで、毎日勉強を続けた。
Determined to pass the university entrance exam, she kept studying every day.
一度こうすると思い込むと、彼はなかなか意見を変えない。
Once he sets his mind on doing something a certain way, he rarely changes his opinion.
3To be unable to stop thinking about something; to be obsessed with an idea.
彼は失敗のことを思い込んで、何日も落ち込んでいた。
He was obsessed with his failure and stayed depressed for days.
一度思い込むと、彼女はそのことばかり考えてしまう癖がある。
Once something occupies her mind, she has a habit of thinking about nothing else.