Regarding your oldest son, I think the best thing you could do is to take responsibility for your behavior toward him. Tell him you realize that you made mistakes, that you didn't treat him with respect. Tell him you're sorry you hurt him. Tell him you want to do better with the younger kids and you're going to do some reading up on it. This is the kind of stuff I'd say.
Then show him and the other kids that you mean it through your actions. If you slip up, acknowledge it and try again. This demonstrates that you're not perfect and you don't expect them to be either. They will learn from your example.
It would have made such a difference when I was younger if my mother had acknowledged what she did. She made excuses and always was very defensive. She seemed to find it next to impossible to admit making any mistakes about anything.
