、。
Even if you fail, don't get discouraged.
Literal
By-chance fail-even-[concessive-ても], be-discouraged don't-[prohibitive-な].
~ても ('even if') is the concessive conditional — 'even if X, still Y.' Distinct from plain たら/ば which expresses 'if X, then Y.' 落胆するな uses the blunt prohibitive な ('don't X'), the terse command-form that says 'do not.' Together, a firm encouragement with rough edges.