I want to express that it is my fault for not understanding something, would it be:
いえいえ、理解しなかった私が悪いのです
or
いえいえ、理解していなかった私が悪いのです
if there is a better way to say this in general please let me know as well!
I want to express that it is my fault for not understanding something, would it be:
いえいえ、理解しなかった私が悪いのです
or
いえいえ、理解していなかった私が悪いのです
if there is a better way to say this in general please let me know as well!
The latter (理解していなかった私が悪い) is usually more natural. I assume something bad happened recently, and it was because of your failure to understand something, which happened even before that. Let me reuse the same image I posted in this question.
In your case, your main concern is probably the time when the bad outcome happened in the recent past (the eye icon in the bottom image). The "point of change" refers to your earlier opportunity to understand something (although nothing actually changed in your case). Saying 理解していなかった can express this time sequence.
On the other hand, 理解しなかった私が悪い is also okay, but this is a simple description of the past; your focus (the eye icon) is your past chance of understanding something. In the context of reflecting on the bad outcome, this may sound slightly sudden unless you clearly say あの時 or something as well.
In other words, 理解していなかった is like "I wasn't aware (when I finally made the mistake)", while 理解しなかった is like "I didn't learn it".