1To be convinced (that); to be under the impression (that); to assume (that) — often with a sense of believing something firmly, sometimes without sufficient evidence.
彼はみんなが自分を嫌っていると思い込んでいる。
He is convinced that everyone dislikes him.
あの人は自分は特別だと思い込んでいるようだ。
That person seems to be convinced that they're special.
私は彼が成功すると思い込んでいたが、事情は違った。
I was under the impression that he would succeed, but things were different.
2To make up one's mind; to set one's heart on; to be determined (to do something).
彼女は歌手になると強く思い込んで、毎日練習した。
She was determined to become a singer and practiced every day.
夢を叶えると自分に言い聞かせて思い込んだ。
He told himself he would fulfill his dream and convinced himself of it.
3To be obsessed (with a thought, idea, etc.); to keep thinking about something obsessively.
彼は過去の失敗を思い込んで、前に進めないでいる。
He is obsessed with his past mistakes and cannot move forward.
その噂を真実だと思い込んでしまい、人間関係が壊れた。
They took that rumor as truth and it ruined relationships.