How does obsessive whispering affect the actions of the heart, especially when it comes to feelings of disbelief and mockery, and make one unable to distinguish between their faith and sincerity and the whispering?

The questioner is suffering from severe obsessive thoughts (waswas), which affect her acts of worship, perceptions, thoughts, faith, and beliefs. This is an indication of her faith, and Allah has forgiven what she has thought to herself. She dislikes these obsessive thoughts, her heart is at peace with faith, she adheres firmly to religion, and performs her obligatory duties. She must cast away these thoughts, not dwell on them, persist in supplication (du'a), fortify herself with beneficial knowledge, ponder the Qur'an, remember Allah, and seek righteous companionship. The cause of these obsessive thoughts might be the evil eye, envy, jinn possession, sins, or psychological reasons. Therefore, she should perform ruqyah (Islamic exorcism), repent, and consult a psychiatrist.