Never say "I'm sorry, but....". Either you are sorry or you're not, no "but" anything....that cancels the whole apology.
Here are some quotes I found that are by people that are much better than I am at expressing themselves.
“Sincere forgiveness isn't colored with expectations that the other person apologize or change. Don't worry whether or not they finally understand you. Love them and release them. Life feeds back truth to people in its own way and time.” Sara Paddison
"Forgiveness does not change the past, but it does enlarge the future"~Paul Boese
“A stiff apology is a second insult…. The injured party does not want to be compensated because he has been wronged; he wants to be healed because he has been hurt.”~ Gilbert Keith Chesterton
"An apology is the superglue of life. It can repair just about anything." Lynn Johnston
"It will never do to plead sin as an excuse for sin, or to attempt to justify sinful acts by pleading that we have an evil heart. This instead of being a valid apology, is the very ground of our condemnation." Archibald Alexander