The ending scene is too heartbreaking, that I have only watched until the end one time.
The final scene is too hard to watch that I have only watched the whole movie once.
The last scene is too painful that I have only watching the movie from start to finish one time.
「最後のシーンが辛すぎて最後まで観れたの1回しかない」は英語に訳すとこのようになります。
The ending scene is too heartbreaking, that I have only watched until the end one time.
The final scene is too hard to watch that I have only watched the whole movie once.
The last scene is too painful that I have only watching the movie from start to finish one time.
最後のシーン→ the last scene/the final scene/the ending scene
辛すぎる訳：
too heartbreaking → 心が辛い
too painful → 辛すぎる／痛すぎる
too hard to watch → 見るのが辛すぎる
（映画を）最後まで観るの訳：
to watch until the end → 終わりまで観る
to watch the whole movie → 全部観る
to watch the movie from start to finish → 始まりから最後まで観る
ご参考までに。
アメリカ合衆国 
