lose one's nerve /luz wʌnz nɜrv/ /luːz wʌnz nɜːv/
to panic /ˈpænɪk/ /ˈpænɪk/
to freak out /frik aʊt/ /friːk aʊt/
lose one's cool /luz wʌnz kul/ /luːz wʌnz kuːl/