1- Don’t doubt your faith. These are satanic whispers to weaken you. Ignore these thoughts as best as you can, and when you get them: say to yourself “I am a believer and I will ignore these doubts.” Then say to Shaytan: I will not let you weaken my faith. I will ignore the doubts you are creating for me.
2- Say everyday la ilaha illa Allah 100 times
3- Wash your head with sidr water
4- Strengthen your certainty in Allah. For example, I recommend seeing such videos: