"I was so sad that I could not stop crying."
Could not - was not able to. (He was pressing the brake pedal, but he could not stop the car.)
or
"I was sad and not able to stop my tears."
Used to express your sadness and tell how sad you were.
"I was so sad that I could not stop crying."（とても悲しくて泣くのをやめることができなかった。）
Could not = was no able to（～をすることができなかった）
He was pressing the brake pedal, but he could not stop the car.
（ブレーキを踏んだが車を止めることができなかった。）
"I was sad and not able to stop my tears."
（とても悲しくて涙がとまらなかった。）
この文章はあなたがとても悲しかったことを伝えることができます。
Both of these examples let the listener know how upset/sad you were and how hard it was for you to stop crying. You can go into more detail if you wish for the listener to know why you were so upset and how it made you feel,
I was overwhelmed by sadness and couldn't stop crying
If you are 'overwhelmed' by something, it means that this thing was so strong you could not resist it or defend yourself against it. In this case, it is a powerful emotion.
This might not be the usual way to say it, but it's correct nonetheless.
It basically means to cry hard for a longer period of time.
Here's how to use it in a sentence:
A: Don't watch this movie if you don't want to be sad.
B: I've already seen it. I bawled my eyes out at the end of it.
これは普通の言い方ではないかもしれませんが、間違いではありません。
"bawl one's eyes out"は簡単に言うと「長い間泣き叫ぶ」という意味です。
以下、会話例です。
A: Don't watch this movie if you don't want to be sad.（悲しい気持ちになりたくないならこの映画は見ない方がいいよ）
B: I've already seen it. I bawled my eyes out at the end of it.（もう見たよ。最後は大泣きだった）
If you are feeling sad then you could say 'very sad' or 'so sad another term for this is 'depressed' if you are very emotional then you tend to cry alot and sometimes feel as if you can't stop
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