How do I use mirroring without it duplicating parts I've already made

So my object is going well, I’m using planes to create it, the issue I’m having is that I’ve had to apply the mirror early because it was starting to glitch out (I couldn’t adjust faces, the vertex would move but the planes didn’t change). Now, I seem to be stuck with it duplicating the faces when I turn the mirror modifier on. It’s a little annoying. Is there any way to turn the mirroring back on, without duplicating what’s already been done?

Cut your model down the middle and delete half. It’s good to have a center line so you can do this easily


Along with using a center-line to control a Mirror Modifier as @joseph said…I also use empties as the Mirror Object when I end up out of the range of the center line, or if I don’t want the mirror to influence something else in the scene…