「、！」。
'Wahahahaha, don't you worry!' I deliberately put on a brave front.
Literal
''Wahahahaha, worry don't!'' I [topic-は] deliberately bluff-[object-を] stretched.
A literary first-person narration moment. あえて ('deliberately, daring to') is a formal adverb marking a conscious choice often against natural inclination. 虚勢を張る (きょせいをはる) is an idiom literally meaning 'to stretch out false power' — putting on a brave front, bluffing strength you don't feel. 心配するな is the plain negative imperative (attach な to dictionary form), the rough/masculine 'don't X'.