Is a person considered an infidel if doubts about faith become settled in their heart, and how can one get rid of self-whispers after sincere repentance from sins?

Waswasa (obsessive whispering from Satan) is one of the most dangerous diseases that can corrupt a servant's religious practice and worldly life. To treat it, it is recommended to continuously strive against these whispers, disregard them, pay them no heed, seek refuge in Allah from the accursed Shaytan, and say: "I believe in Allah," then stop. Know that your dislike and aversion to these whispers are signs that they have not settled in your heart, and that you are on good footing as long as you strive against them.

And if death overtakes you in this state, it will not harm you, Allah willing. Rather, continue in your struggle and supplication until Allah removes that from you.