I recommend the following:
1- Always remember how great and merciful Allah is. You have the best lord, and He gives us many chances. Even if we have made mistakes, as long as we admit our mistakes and seek repentance then He accepts us.
Please see this if you have time:
2- Increase your knowledge. One of the most effective tools to strengthen your faith and Aqidah is to seek knowledge. The more you do so, the less waswasa you will encounter:
Alhamdulillah these days there are a number of online Islamic seminaries that you can benefit from. For example, we have one at: hujjahseminary.com
3- Say “La ilaha illa Allah” 100 times a day
4- Pray Salat al-Layl (at least once a week)
May Allah bless you