How can a person deal with secret thoughts, and is every thought considered a sin, and what should one do if it is, and how does one apply the Hadith: "Indeed, Allah has pardoned my nation for what they merely think to themselves"?

What occurs in the mind but is not acted upon or spoken about is forgiven, by scholarly consensus, as long as one does not settle upon it and persist in it, such as fleeting thoughts of disbelief or backbiting; one is not held accountable for them. It is obligatory to direct one's thoughts to what benefits in religion and in the world, to avert thoughts that have no benefit, to occupy oneself with the truth, and to seek Allah Almighty's help. Worries and psychological disturbances are detrimental to health.