If you owe somebody an apology, time is the interest rate. Pay it soon.

The longer an apology sits, the more it has to account for. What started as one mistake becomes the mistake, plus the silence afterwards, plus everything the other person concluded in the meantime. The original thing might have been easy to settle, but delay keeps adding to the amount owed.

Deal with the smaller version today rather than the larger version next year. Skip the preamble about why you left it so long, since that part is for your benefit rather than theirs. Short, direct, and no conditions attached.