The Supplication of Yunus
لَا إِلَٰهَ إِلَّا أَنْتَ سُبْحَانَكَ إِنِّي كُنْتُ مِنَ الظَّالِمِينَ
La ilaha illa anta subhanaka inni kuntu minaz-zalimin.
There is no god but You. Glory be to You. I have certainly been among the wrongdoers.
What it means
Yunus said this inside the darkness of the whale, in the sea, at night. It contains no request at all, only tawhid, praise and an admission of fault, which is why scholars point to it as the pattern for asking: establish who Allah is, be honest about yourself, and the need speaks for itself.
When and how
Say it repeatedly when you are overwhelmed, especially when you know your own choices played a part in the situation.
Its virtue
The Prophet ﷺ said that no Muslim ever supplicates with the words of Yunus for anything except that Allah answers him, reported in Jami at-Tirmidhi 3505. The Quran itself says that Allah answered him and rescued him from distress, and that He saves the believers in the same way.
Source
Quran 21:87
Quranic text from Tanzil.net, Creative Commons Attribution 3.0.