What are two things that cause us to doubt ourselves, according to Emerson?

Two things that cause us to doubt ourselves are not being true to the inner voice of our souls and failing in our first endeavor.
Emerson contends that we doubt the divine spark that God has placed within us if we constantly look to the past or the future for inspiration. He advises young men to throw away their books and live in the present moment. He points to plants, saying they are not looking to the past but being true to themselves as they grow and blossom. He writes the following:

Thus all concentrates: let us not rove; let us sit at home with the cause. Let us stun and astonish the intruding rabble of men and books and institutions.

Likewise, while we need to care for our family and friends, we cannot allow them or their ideas of what we ought to do shake us from our true path: 

Say to them, O father, O mother, O wife, O brother, O friend, I have lived with you after appearances hitherto. Henceforward I am the truth’s.

Emerson also writes that,

If our young men miscarry in their first enterprises, they lose all heart.

Instead, he says, we should brush off failure, hop up, and try again. The world is a big place and full of opportunities. If we have not found our destined path on the first try, there are a hundred more paths to explore. The key is to keep trying and to have faith that we will eventually succeed.