"Let me hear what God the Lord will speak,
for He will speak peace to His people, to His saints;
but let them not turn back to folly.
Surely His salvation is near to those who fear Him,
that glory may dwell in our land."
I've heard Joyce Meyer say that maintaining our peace is one of the highest forms of spiritual warfare. The devil is actively out to steal our peace. I believe living a peaceful life comes from living a life fully trusting in God and His will for our lives. Trust can be extremely difficult especially when we wonder "if You are God of the Universe, why did you allow this or that to happen?"
I don't think we'll ever understand the answer to that question on this side of eternity, but I do know He promises to make all things new (Rev. 21:5) and to work all things together for good to those who love Him (Romans 8:28). So each day, we press on, trusting Him and maintaining the peace He has given us.