The fast is only nullified when a substance enters the body cavity through a recognized point of entry, such as the mouth, nose, or front and back private parts. Therefore, eye drops, injections, and nicotine patches do not break the fast as they do not enter the body from a recognized point of entry.
As for water entering one’s body cavity, one’s operational certainty is that one is fasting and that hisfast is valid. Unless one is absolutely certain that something entered a recognized point of entry and reached his body cavity, the fast will remain valid. As for misgivings and doubts, these should be ignored as they are merely whispers from the devil, following which is sinful. Imam Kasani in his Bada`i al-Sana`i states that if one is plagued with devilish whispers regularly, he should not pay any attention to it whatsoever. Our religion has been made easy for us, so we should not overburden ourselves. Rather, we should suffice with what the Messenger of Allah (Allah bless him and grant him peace) taught us.