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You might not have known the rules, but you certainly did know that you were copying other artists work and chose not to mention it, knowing that your failure to mention it would result in curators assuming it was your original drawing , and that is what matters.
Additionally:
Ignorantia juris non excusat.
https://en.wikipedia.org/wiki/Ignorantia_juris_non_excusat
It is precisely when all the truth is revealed that you will be able to be in peace. And for that, we have a way to go.