I'm ashamed at how disrespectful I was when I was 15.
What made you be disrespectful at 15?
I thought my parents weren't treating me fairly and I was upset with them.
You should respect your parents so that they will have a more positive view of you as a person. Being disrespectful to them will only make them not want to hear your feelings or concerns.
I know, I've learned from my mistakes.
It's great that you've realized that you weren't acting appropriately when you were younger. There is always time to change, and show your parents the respect that they deserve.