When singing through a PA or in a studio, I find that what I hear is a voice coming from somewhere else, disembodied and seemingly nothing to do with me. So my voice may wander out of tune, and I can't tell if it's sharp or flat, or what on Earth to do about it! So, have none, or very little, of your voice in the monitors, plenty of everything else, so you can hear what you're singing to, and give it all you've got! If you have difficulty hearing yourself at all, stick your finger in your time-honoured ear (one headphone on, the other moved out of the way. It works for me and others.