There are things that we don't know.
We don't know, for sure, why we are here. We don't know what happens after death. There are many times that we can be unsure about the best course of action for us, or about what we are "meant" to do, or about how we can make a difference in the world, or at work. And there are times that because of this uncertainty, we are plagued with doubt.
Doubt isn't a problem because we don't know things. Doubt is a problem because we think we should.
Try a thought experiment. The next time you are feeling frustrated with not knowing an answer to a big question, say "I don't know, and, right now, it seems that I am not supposed to know."
Cut yourself a break. The list of things that we know is short. The list of things we do not is long. There is a space of not knowing, and knowing that you cannot know, that can be open, nonjudgmental, and even comforting.
When you really need to know (as opposed to just wanting to), you will.