1)
Why did he say "Avonosai Avru Roshi"?
1.
Radak: He depicts his sins like great water that passed over his head, and he is sinking in it.
2.
Malbim: Avon is crooked thoughts, i.e. Mezid. It is attributed to the head.
2)
Why did he say "Chaved Yichbedu Mimeni"?
1.
Radak: He depicts his sins like a burden that is too great for him to bear. I.e. I do not have merits to lighten the sins.
2.
Malbim: They are a great burden to me amidst my great regret and sadness over them.
