Recite sura naas when ever u have doubts the doubts will fade away. If by reciting once does not help recite once more. It works with me and many have tried, the doubts of Shaytan fade away.
Food be carful of what you eat. Eat as much as halal food you can.
Avoid doubtful food and recite this Dua in your Salaat at least once
Daily in your qunoot
Holy Quran 3:8
رَبَّنَا لَا تُزِغْ قُلُوبَنَا بَعْدَ إِذْ هَدَيْتَنَا وَهَبْ لَنَا مِن لَّدُنكَ رَحْمَةً ۚ إِنَّكَ أَنتَ الْوَهَّابُ
[Who say], "Our Lord, let not our hearts deviate after You have guided us and grant us from Yourself mercy. Indeed, You are the Bestower.