The English translation was "We talked it over, and we realised we were both just confused." But I cannot understand "思い込み" here. The speaker and a woman were about to become lovers (付き合う). But after some happenings and talking, he discovered that actually some other boy was more important to her life than he (speaker), and he (speaker) did not really love her that deeply but was attracted to her because she was very kind and gentle to him.
Judging by the context, I guess that the word would mean something like 勘違い, 誤解, "wrong perception", "misconception" or something like that, but if I look up dictionaries, the meanings were "deeply believe" or "firmly decide".
PS: If you need more context, see the transcript at http://anicobin.ldblog.jp/archives/56894558.html . You can use Ctrl+F to find the position where that sentence was said.