Verb (5-dan, む, intransitive)
1. to be convinced (that); to be under the impression (that); to feel sure (that); to assume (that)
あって当然だと思い込んでいた。I had just assumed it would always be there.
2. to make up one's mind; to set one's heart on; to be determined (to do); to be bent on
こうと思い込んだら、彼は後には引かない。Once he sets his mind on something, he never backs down.
3. to be obsessed (with a thought, idea, etc.); to think about obsessively