Hi Mi Yeon,
'a talk' is natural, like you talked about something,
but 'a conversation' seems organized, like a set topic from a teacher in school: 'today's topic is..'
:)
'there's nothing I can do but..' or 'there's nothing for it but..' are ok, but
'I can't help (myself) but ..' is a different meaning, like it's your automatic action that you can't prevent!
:)