at ease · to feel relieved · to set one's mind at rest · to keep one's mind on sth

Let me work in peace.

With the T.V. on, how can you keep your mind on your studies?

Yea, though I walk through the valley of the shadow of death, I will fear no evil: for thou art with me; thy rod and thy staff they comfort me.

A good warranty would make me feel better.

My mind is at ease believing you'll keep the secret.