This is a bit old but relevant. In this post, John P. Forsyth Ph.D. states:
Peace of mind is something we all seem to want, and want more of. Few of us get it, and when we do it tends to be fleeting. I think the reason has something to do with how we think of “peace of mind.” It is not something we can have and hold, but it is certainly something that we can learn to cultivate and allow to grow.
He then offers a simple little exercise to help cultivate peace of mind.