Is saying "but" (meaning: however) after the sentence "Perhaps it was good that you didn't go on your trip" considered an affirmation that Allah chose for me not to go, and that this contradicts my actual going, thereby falling under the danger of saying "Allah knows" when it contradicts reality, as mentioned by Sheikh Ibn Uthaymeen?

We ask Allah to grant you well-being from obsessive whispers (waswas (definition)) and lying. Lying is forbidden (haram (definition)) and one must repent from it. Regarding saying "Allah knows" falsely, it is a serious matter and more forbidden (haram) than lying, but it does not amount to disbelief (kufr) according to the majority of jurists. However, if the speaker intended that Allah is ignorant of the matter, or knew that his statement was an act of disbelief and still deliberately uttered it, then he might fall into disbelief. The situation you mentioned does not fall under this category; it is due to the influence of waswas, which you must ignore.