I had no serenity in early recovery, so I prayed for it.
The Serenity Prayer is a request to God for serenity. It is not a mantra. You cannot talk yourself into serenity. It is a gift that is given to you when you work the 12-Steps and with humility and accept life on life's terms.
According to David Richo the terms are as follows:
You are alone. Things change whether you want them to or not. Life is full of suffering. Life is unfair.
Most of my serenity comes from acceptance and positive thinking.