An apology that ends with “but” is just an argument wearing a costume.

A qualifier tacked onto the end cancels everything before it, because it signals that the real sentence was always the second half. What the listener hears is a defence with a courtesy attached, and now there are two things to be annoyed about. A genuine apology has no explanation bolted on, which is what makes it uncomfortable to give. The discomfort is roughly the point.

Next time you owe one, say what you did, say it was wrong, and stop talking. Let there be a pause. If there is context they genuinely need, offer it the following day as a separate conversation.