"Sorry" is an adjective that expresses regret or remorse. It does not describe a quality that can be compared or ranked in terms of degree.
You could express different levels of regret using other words or phrases:
* More regretful: "I am deeply sorry," "I am truly sorry," "I am very sorry."
* Most regretful: "I am incredibly sorry," "I am profoundly sorry."
But, you wouldn't say "sorrier" or "sorriest."