▲思い込みであなたが来月写真を撮影しに行くと言ったと勘違いしてた



be under the wrong impression that　を使っていいのでしょうか？

i was under the wrong impression that you said you are going to take a picture next month.

思い込みでって日本語でもあまり使わないような？
勘違いしてた、だけで充分では？

I was misunderstanding that you said you are going to take a picture next month.
　

この場合はMisunderstandingを使えばいいのですね（笑）
そうかもしれないですね、思い込みでって変かもしれません・・・
教えてくれてどうもありがとうございました！勉強になります！ 　

>I was misunderstanding

I misunderstood ... 　

↑Right.
With such verbs as know and understand, you don’t usually
use the progressive form, such as:
I am knowing, or
I am understanding,
unless you are actually in the process of understanding
something, even so you usually say,
I’m trying to understand...