All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
Thihaar is when a man renders his wife forbidden for him by saying: “You are like my mother to me”; i.e. “You are unlawful to me exactly like my mother”).
As for what you mentioned in your question, which is writing the expression: “What is Thihaar?”; then this does make Thihaar effective.
What will help you to get rid of these whispers is to have trust in Allah, rely on Him, seek refuge in Him, and mention Him and supplicate him as much as possible, but do not pay any attention to these whispers.
For more benefit, please refer to Fataawa 375022 and 306568.
Allah knows best.