I was suffering emotionally for a long period of time.
ーI was in the dark for a long time mentally.
「長い間精神的に暗闇の中にいた」
to be in the dark で「暗闇の中にいる」
for a long time で「長い間」
mentally で「精神的に」
ーI was suffering emotionally for a long period of time.
「長い間心的に病んでいた」
to be suffering で「病む・患う」
emotionally で「心的に・情緒的に」
for a long period of time で「長い間・長期間」
ご参考まで!