How can one definitively get rid of obsessive thoughts in acts of worship, especially if they decrease one day and increase and intensify another, and is it possible to be completely cured of them?

The treatment for intrusive thoughts (waswas) is to ignore them and not pay them any mind. If you realize that what is afflicting you is waswas, then turn away from it and do not give it any attention, until Allah blesses you with complete healing.