Does fasting become void if water enters the stomach unintentionally while exaggerating the sniffing of water during Wudu' (ablution)?

First: You did well by not exaggerating in sniffing water up the nose (istinsḥāq) during ablution when not fasting, as the Prophet, peace and blessings be upon him, said to Laqīṭ ibn Ṣabirah: "Perform ablution thoroughly, intersperse water between the fingers, and exaggerate in sniffing water up the nose unless you are fasting."

Second: If water enters the stomach unintentionally while gargling (madmaḍah) or sniffing water up the nose (istinsḥāq) during fasting, it does not invalidate the fast, as Allah Almighty says: "And there is no blame upon you for that in which you have erred, but [only for] what your hearts intended."

Third: The cure for intrusive thoughts (waswasah) is to turn away from them and not pay attention to them, while frequently remembering Allah, obeying Him, and avoiding His disobedience.