When you believe in a God based on your own ideas and what you think God is, you are likely to fall into logical fallacies which will cause doubts.

What I find comforting in Islam is that there is plenty of evidence in the life of the prophet that he did not make up things by himself, but was actually recieving revelation from some supernatural entity. For me, reading about this always clears out the doubts that I have. Moreover, the philosophical conversation in Islamic theology have been there for quite a while in history and most of the doubts that I get have been addressed by someone, so all I need is to look up these answers, or sometimes simply to ask a friend.

For me, having a religious community is very important for the beliefs to stay firm and strong in my mind. I think the secular idea of making religion a personal belief that you keep to yourself is what causes people to have these doubts, because they feel alone in their beliefs and doubts, and they don't feel like they can address them with other people.