There are many types of self doubt, both good and bad. A good self doubt raises questions about ones character and nature.
The heart has very deceitful tendencies so to question one's motives is helpful at times.
What is unhelpful is to question one's standing before God on the basis of what one finds in ones heart.
The reality is that through faith Jesus acts as our righteousness. If we are looking to our own righteousness as justification of our standing before God we have errors.
A Christian will always sin, but the desire is now turned toward serving God. The desire to serve God is one of the marks of a Christian.
Another is loving ones neighbor whoever that is. We should not be surprised when we find man people in the church who do not love God or their neighbor.
Attending church does not produce faith in God. Rather it is a correct relation to God which makes one a Christian through the Spirit of God uniting us to the work of Jesus.
Often we will people with no self doubt in the church. This is an error. But on the other side we will find people with nothing but self doubt when they should instead look to Jesus.