Making amends to an ex after years since breakup

My ex broke up with me in 2006 after 5 years together. I did take her for granted and there was infidelity on my part. In 2008, when I was feeling low, I reached out to her and she said she was seeing someone, I just wanted to talk. We did talk over the next few months, but it was me who was always calling her. In early 2009, I figured not to call anymore and there was never any more contact. In the ensuing 12 years, I have changed and taken a long hard look at myself and was thinking of calling her, leaving a message about how I should have treated her better and looking to amend my actions. Would this be appropriate? Has too much time passed? How would you feel if an ex called and left a message after so long? I do realize we will never be together.

Thank you