If you look back at the parable of the lost son, what would have happened if the lost son had decided to not return? What if he had simply felt bad about it and never physically went back to his father? Would he have had the welcome feast? Would he have been forgiven? No, in fact nothing would have changed for him.
Repentance is the same thing, we must go to the father in prayer and ask God to forgive us of our sins with a repentant heart.