Well - the fact you are capturing anything means the mic is powered, so it's working well. If you are describing a decent enough capture but the headphone audio the singer is singing to is leaking out, and being picked up by the mic - then the solution is to swap to headphones that don't do it. What are they? Keep in mind some headphones are closed - as in the audio ONLY comes out of the seal between the pads and your head, with this seal being on the skin around your ears and NOT them sitting on your ears. Others are open back where the rear of the headphones is open to the room, and of course these leak too. Some really good headphones leak like sieves sealing and isolation are not important in the design. If you tell us the make and model, we can double check - but if the sound is escaping, the mic is not to blame for capturing it. You could try in ears - where they have a good seal between the in-ear transducer and your ear canal - they're usually very good for isolation. I suspect you just have open back headphones. When I have real problems with a very quiet voice and a loud track, I give people some drummer headphones that clamp your head like a vice, which seal really well - BUT - hurt like hell. OK for short sessions.