1st thing in every practice, last thing in every practice, 1st thing in every lesson: play/sing the song straight through without breaks. Don't stop and restart for mistakes or weak places. Keep the rhythm exactly correct, whatever the number of measures in the song, when you've counted that many measures, the song should be over. Stop. This is how performing really is, so practice this way (but not only this way). 1/3 of your practice time on the song should be done this way.
It is a wonderful and momentous thing when you play or sing and you realize you're actually doing it, you're really making music.