Fetching details from the dictionary...

to be convinced (that); to be under the impression (that); to feel sure (that); to assume (that)
to make up one's mind; to set one's heart on; to be determined (to do); to be bent on
to be obsessed (with a thought, idea, etc.); to think about obsessively
彼女は自分が正しいと思い込んでいる。
She firmly believes that she is right.
勝手に思い込まないでちゃんと確認して。
Don't just assume; confirm properly.
彼は失敗したと思い込んでしまった。
He convinced himself that he had failed.