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こんでいる。
She is convinced that she is right.
道を間違えたと思いこんで引き返した。
I turned back, convinced I had taken the wrong road.
一度思いこむと人の話を聞かなくなる。
Once he gets something in his head, he stops listening to others.