How does a brain contusion heal itself? Why didn't I feel different?

With time. A brain contusion is caused by head trauma or a rapid sudden 'whipping' of the head. If some blood vessels bleed or bruise it will cause some local swelling. There is often headache, nausea, some lightheadedness. But there can be no symptoms or severe neurological symptoms or problems that is related to the location of the contusion and the severity of the contusion.
Location, atrophy. Depending on the location, a contusion may not disturb the brain to a point that you can sense it. Also, the very elderly brain has lost a lot of substance over the years (atrophy/'shrinkage") and is quite loosely packed inside the skull. Even a big bruise and swelling can be absorbed because there is so much "wiggle room" inside the head.