Sometimes it is natural for us to have doubts, and it doesn’t mean we are weak in faith. It’s normal to have doubts, but what’s dangerous is to leave the doubts unaddressed. Resist those doubts by seeking knowledge. The more you seek knowledge the firmer you will become and the less you’ll impacted by doubts.
Take seminary classes (many are available online). Courses like Islamic Beliefs, Usul al-Kafi and Nahjul Balagha are really helpful. We offer some of these at hujjahseminary.com.
Read this Dua daily:
بسم الله وبالله، توكَّلت على الله، أعوذ بالله السَّميع العليم من الشيطان الرَّجيم
And I also recommend seeing this lecture: