"I am sorry."
pardon in esperanto actually means "forgive." when you say Mi pardonpetas, you are saying "I ask for forgiveness." Another version would be "Mi bedaŭras", which means "Regret dawns on me". I don't think there's an esperanto word for "sorry", although I could be wrong. I hope this was helpful!
I think the definitions should be more clear that "pardoni" = "to excuse" while "pardonpeti" = "to ask-for-forgiveness a.k.a. to be-sorry" and "bedaŭri" = "to feel-sorry". Even though the concepts are often mixed in English, they are different in most languages. So probably keeping pardoni separate from the other two would create a lot less confusion.