You sound unsure of yourself when you sing. Also there's rhythm issues with the song. Listen to someone like Whitney Houston or Martina McBride sing the
song and tap your foot along to what they sing. Try singing along. Choppy rhythms can make you sound unsure because it makes the song sound awkward. Like, I
may know the melody, and the words, but something is off. People may not know what is off but subconsciously they feel it.