Confidence and Doubt

It's safe to say that we are at least somewhat confident in what we believe, or we wouldn't believe it.

However...

Considering your belief system/worldview, what do you doubt?
What do you struggle with?
Do you ever think "is this really true"?
How do you deal with it?

If you have doubts, how do you reconcile them?

what amount or degree of doubt would lead you to abandon your worldview