Doubt is a logical process for someone who wish to know what is true and don't stay captive of wrong traditions. I don't know someone who believes in God and at least one time in it's life didn't doubt about God's existence, Bible's inspiration, his denomination truthfulness. Many people in the Holy Scriptures had doubts. From Moses to Jeremiah and in NT Thomas. The first two had doubts about themselves, "am I the correct person for this appointment?" and this was a logical process to convince themselves. In the other had Thomas doubted Jesus. He asked more proves a logical process until convince his mind that the man before him was Jesus.
About your question:
I have trouble accepting the Christianity is the true religion of God
when there are all these other differing religious opinions in the
world. How do we know which one is true?
Here is what helped me. Let's say you want to find the height of a door. You asked someone near you "What is door's height?" he looks at it, and say's 2.30. You ask another says 2.50, someone else 2.10. The best way to find the height is simple: the meter. In the same way the meter for Christian truth is the Bible. God in this book helps us to understand what people do his will nowadays. For example ask yourself the following questions and try to ask honestly:
What denomination base its teachings on the Bible? 2 Timothy 3:16,17 & 1 Thessalonians 2:13
What denomination worship only God and make his name known? John, 17:6, Matthew 4:10, Psalms 83:18
What denomination show genuine love for one another? John 13:35, Colossians 3:14, 1 John 3:10-12; 4:20, 21.
What denomination accept Jesus as God’s means of salvation Acts 4:12, Matthew 20:28, John 3:36
What denomination is no part of the world? John 18:36, Acts 5:29 & Mark 12:17
What denomination preach God’s Kingdom as man’s only hope? Matthew 24:14, Psalms 146:3, Matthew 6:10, Daniel 2:44
True Christians, in my view, fulfill all the above requirements that Bible sets. If you have honesty be sure that God will drive you in the correct path.