I'm sorry I wasn't good enough.
I'm sorry I was too perfect. 
I'm sorry I was too sick.
I'm sorry I never told anyone.
I'm sorry no one believed me.
I'm sorry we grew apart.
I'm sorry I was condescending.
I'm sorry I made a mistake.
I'm sorry you couldn't forgive me.

But most of all,
I'm sorry I'm not really that sorry.