It's even hard for someone with a mental illness to recognize their illness, so it's even more difficult for someone else to know that another person is mentally ill.
ーIt's even hard for someone with a mental illness to recognize their illness, so it's even more difficult for someone else to know that another person is mentally ill.
「精神病を抱えている人は、自分でも病気に気がつくのが難しいのだから、他人がその人に精神病があるかを知るのはもっと難しい。」
someone with a mental illness で「精神病を抱えている・患っている人」
recognize「気がつく」
be mentally ill「精神を患っている・精神障害である」
ご参考まで！
カナダ 
