you treat the matter lightly, where

you treat the matter lightly, whereas I myself was never more serious"

Results (Chinese) 1:

你掉以轻心，而我自己是从来没有更严重”

Results (Chinese) 2:

你轻描淡写地对待这件事，而我自己从来没有更严重过。

Results (Chinese) 3:

你对这件事轻描淡写，而我自己却从来没有这么认真过。”