Are both expressions correct in this kind of sentence or is it only possible to use confident? What's the difference between them in this sentence?
Though I knew the song by heart, I wasn't feeling confident / self-confident.
Self-confidence is being secure in yourself and your abilities. This is different than being confident, which draws on evidence of things you've been able to do or achieve in the past. So its better if the sentence is
Though I knew the song by heart, I wasn't feeling confident.
Source: Desi Creswell