HELP! How to mirror half of a rigged model, and have it still rigged?

[The following images are a re-creation of my situation. The model used in these images is symmetrical and I know that. Don’t say “why do you need to mirror it?” because the model I originally had the problem with (and I still do have the problem) is not symmetrical so that is why I need it mirrored]

I have a model with two sides.

One side I don’t like, So I deleted all the faces on that side.

(Mesh > Delete > Delete Faces)

Next, I added a mirror modifier, and apply it.

(Modifier tab > Add modifier > Generate > Mirror)

It looked like this

So, I haven’t mirrored the armature, and that’s what I’m having trouble with. I want the new mirrored side of the model to be controlled by the old armature system underneath. I don’t know how to do this, though.

Now when I bend the armature underneath the new side of the model it looks like this:

I know why this is happening. I just don’t know how to make it work.

If anybody can help that would be awesome. I have had this problem for days and I need to get it solved asap!

If you apply the mirror modifier it should transfer the “right” vertex weights to the new geometry (i.e., upperarm.R, leg.R, etc.).