There are lots of people who mistake their imagination for their memory.

Memory is reconstruction rather than playback. Each time you recall something you rebuild it, filling gaps with what seems likely and quietly folding in later information, so a vivid recollection can be substantially invented while feeling completely solid. Confidence is no guide here, since the strongest memories are often the most rebuilt.

In a disagreement about what happened, hold your version more loosely than feels natural. Look for something written at the time, a message, a note, a receipt, and treat that as more reliable than your certainty. Where nothing was recorded, allow that both of you may be honestly remembering different things.