"K'rov rachamecho m'chai p'sho-ai "
"In accordance with Your abounding compassion, erase my transgressions".
We are asking Hashem to forgive us and we are SO sure that He will, therefore we say the brocha of;
Boruch ata Hashem chanun hamarbe l'sloiach.
One who pardons abundantly.
(i like that word.)
Sofek brochos l'hokel.
We are forbidden to recite a brocha of doubtful obligation, for fear it be in vain.
Were there even the slightest doubt as to whether Hashem forgives the sinner, we would never have been commanded to recite the brocha.
But there is no doubt here whatsoever!
For we have asked "...Forgive us".