、。
Even as I was saying that, I did the exact opposite.
Literal
So say-[while-ながら], I [topic-は] complete-opposite [gen-の] behavior [to-に] came-out.
~ながら 'while / as X' attached to a masu-stem marks a simultaneous action. 真逆 (まぎゃく or まさか) 'the complete opposite / the exact reverse,' where 真 'true / exact' intensifies 逆 'reverse.' 行動に出る '(to) come out into behavior / act' is a set collocation — 'to take action / to act,' specifically when the action is a commitment or a break from contemplation.