I woke up one day and realized that “life is short.”
One day I realized that “life is too short.”
❶One day I realized how short life is.
(ある日、人生が短いことに気づいた)。
❷I woke up one day and realized that “life is short！”
(ある日、起きて気づいたことは、人生が短いってこと！)。
❸One day I realized that “life is too short！”
(ある日気づいたのは、人生は短すぎる！てこと)。
One dayーある日
Realizeー気づく
Lifeー人生
Shortー短い
So we should enjoy every moment of it！
(だから一瞬一瞬楽しんだ方がいい！)。
One day I had the realisation that life is very short.
A 'realisation' is when something becomes apparent or focused in your mind. These usually happen very quickly like an epiphany and can sometimes shock people into changing the way they lead their lives.
For example if someone all of a sudden realises how short life is, they may go travelling and see the world 'before it's too late'.
I just realized that life is so short.
Life is so short and I just realized it.
Things change so quickly and life is so short.
Life is short and I'm reflecting on it.
I just realized that life is so short.（人生が短いということに気が付きました）
Life is so short and I just realized it.（人生が短いということに気が付きました）
Things change so quickly and life is so short.（時の流れは早く、人生はあっという間）
Life is short and I'm reflecting on it.（人生は短いです、今それについて考えています）
If you suddenly think of something then you are said to 'realise' you only live once and it can be short so you should enjoy yourself and make the most of it you can express this as 'life is too short' or 'how short life is'
突然何かが頭に浮かぶことは 'realise' で表せます。
「人生は一度きりでいつ何があってもおかしくないので、悔いがないよう楽しむべき」。これは 'life is too short' または 'how short life is' で表せます。
When you realise something, you become aware of it. Life has always been short, but what you are saying here is that you've suddenly become aware of that. You could of course also say that 'I became aware that life was short', which would mean the same thing.
'It struck me' means that you've suddenly had the realisation.
Incidentally, the word realise (spelt with an s) is just an alternative spelling for realize (spelt with a z). We tend to use realise in British English and realize in American English.
"realise" は "become aware of"（気が付く）という意味です。人生は前からずっと短いわけですが、ここでは、そのことに「はっと気が付いた」と言っています。
もちろん、'I became aware that life was short'（人生が短いことに気が付いた）と言うこともできます。これも同じ意味です。
'It struck me' は「はっと気が付いた」という意味です。
ちなみに、"realise"（スペルが "s"）は、単に "realize"（スペルが "z"）の別のスペルです。イギリス英語では "realise"、アメリカ英語では "realize" が使われることが多いです。
This one day, my eyes opened to the fact that life is short.
One day, I realized that life is short.
Use any of the above when casually talking about life being short.
Life can seem to be a long process, from the many years spent as a child to finally growing up into an adult. When we look back on all the years that have passed we realize that life is, in fact, short, life seems to have flown by.
Epiphany: realization
Flown by: passed very quickly, came and left quickly.
アメリカ合衆国 
イギリス 
南アフリカ 
