Ich tue mich schwer, diese englische Phrase ins Deutsche zu übersetzen. Als temporäre Lösung hätte ich: »Ein ruhiges Gewissen haben.« Allerdings soll das deutsche »Get complete peace of mind« ohne Kontext dastehen, was mit der gewählten Übersetzung irgendwie nicht ganz so passt.
That would be appropriate if the original version would be
Get, however, implies a change of state
That slightly indicates, that the conscience was mollified.
is quite commonly used and there is something of an exaggeration in it.
is kind of profound.
Related phrases are
doesn't completely capture the original as it assumes that a troubled conscience, i.e. your wrongdoing, be the cause of your unsettled mind, while there can be numerous other reasons ( being overworked, economic hardship, emotional stress etc. ).
(as suggested by @embert), though that assumes there has been an inner fight that has worn you down (in common speech though, the recipient probably wouldn't wonder ).
I'd recommend the imho most widely applicable
which also conveys a meditative notion.