Almost everyone is carrying a sentence they never got to say. "I'm sorry." "I forgive you." "Was it my fault?" "I love you" - said a thousand times but not that morning, not at the end, not when it counted. The unsaid thing has a weight all its own.
Here's what guilt is actually made of, most of the time: love, with nowhere to go, turned around and pointed at yourself. You replay the last argument, the missed call, the visit you postponed, because replaying feels like it might still change something. It's the mind refusing to accept that the story is over - which is another way of saying it's the mind still loving them.
You were a person, doing your best inside an ordinary day, with no way of knowing it wasn't ordinary. The standard you're holding yourself to - perfect foresight, perfect patience, perfect goodbyes - is one no living person has ever met.
If the sentence still wants saying, say it. Out loud in the car. In a letter they'll never read (this app has a postbox for exactly that). Words don't need a listener to be true. They just need saying.