Waswasah can drive a person into a vicious circle with no end. To combat it however, you have to let go of your doubts and rule the particular thing or act as clean or correct respectively.
((Would you agree if an impurity on an object cannot be seen, smelled, or felt, then the object is pure?))
To reply to this pls see the link below for a simple example of the same: https://www.sistani.org/english/book/48/2117/
I would also recommend you read the entire topic on Taharah on the following link: https://www.sistani.org/english/book/48/2115/