。
I admit I was careless.
Literal
I [subject-が] careless thing [topic-は] admit.
不注意 ('careless, inattentive') is a negated compound (不 + 注意 'care, attention') used as a na-adjective. こと nominalizes the 不注意な (adjective) + こと ('the fact of being careless') — clause marked with topic contrastive は. 認める ('to acknowledge, to admit') is the admission verb. A mature self-correcting statement.