Praise be to Allah.
Fasting is one of the pillars of Islam, as is well known.
Therefore it is not permissible for the Muslim to be heedless about it just because he feels thirsty or hungry, or just because he is afraid that he cannot fast. Rather he must be patient and seek the help of Allah, may He be glorified and exalted. There is nothing wrong with him pouring water on his head to cool down, or rinsing out his mouth.
He must begin his day fasting, and if it so happens that he is not able to complete it, and he fears that he may die or become sick, it is permissible for him to break the fast in that case. But he should not break the fast on the basis of a mere assumption; rather he should not break the fast unless he experiences hardship.
Ibn Qudaamah said:
The correct view is that if he fears for himself due to extreme thirst or hunger and the like, then he may break the fast.
Shaykh Ibn ‘Uthaymeen said in his commentary on al-Kaafi:
“If he fears thirst” does not refer to mere ordinary thirst; rather it refers to the kind of thirst in which there is the fear of dying, or there is the fear of suffering harm.
End quote from Ta‘leeqaat Ibn ‘Uthaymeen ‘ala al-Kaafi (3/124).
An-Nawawi (may Allah have mercy on him) said in al-Majmoo‘ (6/258): Our companions and others said: Whoever is overcome by hunger and thirst, and fears that he may die, must break the fast, even if he is in good health and is not travelling, because Allah, may He be exalted, says (interpretation of the meaning):
“And do not kill (or destroy) from yourselves: Surely, to you Allah has been Most Merciful (Raheem)”
“and do not throw [yourselves] with your [own] hands into destruction”
And he must make up that day, as is required of one who is sick. And Allah knows best. End quote.
So you must make up that day, and if you were too hasty and broke the fast before experiencing the level of hardship that makes it permissible to break the fast, then you must also repent from what you did, and not do that again.
And Allah knows best.