What is a single word for someone who is easily discouraged? [duplicate]

Someone who is pusillanimous is one that lacks courage or strength of purpose.

pusillanimous, adj. 1. Of a person: lacking in courage or strength of purpose; faint-hearted, craven, cowardly. (OED)

If you want to include an element of reproach in your description you could call them a coward.

coward, adj. 1. A reproachful designation for one who displays ignoble fear or want of courage in the face of danger, pain, or difficulty; an ignobly faint-hearted or pusillanimous person. (OED)

Taking discourage here literally (dis-courage), I suggest fainthearted:

lacking courage or resolution: timid (Merriam-Webster)

Quitters. People that quit doing what they're working on at the slightest bump in the road.

Any person who is timid will not get far in life

ODO:

timid

ADJECTIVE

Showing a lack of courage or confidence; easily frightened.

‘Once inside, things simply got worse for any shy, timid souls who plucked up the courage and made it past the front door.’

A defeatist is someone who very easily accepts failure.

https://en.oxforddictionaries.com/definition/defeatist

defeatist NOUN

A person who expects or is excessively ready to accept failure.

I would work it into your sentence as:

"Any person who is such a defeatist will not get far in life."