Peace of mind is knowing that you do your best to do what is right as often as possible. Peace of mind is knowing that you can never be perfect, but that you can try to be as perfect as possible. If you accept that you will never achieve perfection, you can have peace of mind so long as you know that you are doing your best.
For example, I know that I make mistakes raising my kids. But I know that I'm doing my best and trying hard to do the right thing for them at all times. That gives me peace of mind. (Of course, they're not teens yet. I imagine I'll have no peace of mind then...)