Guilt is stressful enough, without the angst of being treated as a shameful wimp! Be assertive when you have an apology to make.
USE A STRONG, CALM AND EVENLY PACED VOICE - keep the whimpering, pleading tones out of the office and say sorry just once.
EXPLAIN ONLY WHAT'S NECESSARY. Don't be shamed into divulging reasons that have a right to remain private.