Gratias tibi ago.
However, I’m wondering if I haven’t asked the wrong question. I am looking for the closest parallel (with the modified meeting) of the phrase mea culpa, whose common use in English surely derives from the Latin liturgy’s confession:
Confíteor Deo omnipoténti
et vobis, fratres,
quia peccávi nimis
ópere et omissióne:
mea culpa, mea culpa,
mea máxima culpa.
I confess to almighty God
and to you, my brothers and sisters,
that I have greatly sinned,
in my thoughts and in my words,
in what I have done and in what I have failed to do,
through my fault, through my fault,
through my most grievous fault.
In trying to match the grammatical construction of that original (even at the cost of some ambiguity when the phrase stands alone), would it be better to use alia culpua or alius culpa. My naive thought is that the -a ending matches the declension of the pronoun, but I really don’t know anything about Latin declensions, so it’s entirely possible I’m mistaken.