Here is a photo of the whole wall. Thanks

What's your budget look like? I think the proportions are off. I would remove the shelves and reface the fireplace. Add some height to the top of the fireplace and narrow out the sides a bit. A quicker cheaper fix might be to add a chunky mantel on top, may help with proportion a bit. Seems very squatty.

I would rip out the shelves on both sides, build out a new wall above the fireplace and re-face the fireplace with a new stone floor to ceiling on the same plane. Then you could do built ins on both sides Doors could be on bottom with open shelving above

If you like the stone, I would consider painting the stone and the entire wall a warm white.

The stone is not the issue here, it is nice and if you paint it, it will always look cheap. You want some better paint colors, and then, some help making a gallery wall above, and finding one great sculptural item for the mantel centered over the fireplace.

Let me suggest a warm gray neutral for all the woodwork - since the painting has begun, lets get it deeper than the stone so the fireplace area comes forward to the eye. Test bm rockport gray http://www.benjaminmoore.com/en-us/paint-color/rockportgray

Here it is on a wall -

and here is a close cousin (bm river reflections) on some casework .. you could test it too, but I think it has enough green khaki in it that it will make the stone look pink.

Look into what it would cost to get a solid piece of thick gray slate (at least 3 cm / 1/1/2 thick) (not an expensive stone) to top all the painted mantel from left to right - this might be a long term plan to save up for. You might like quartz that looks like limestone (but that is more expensive per lineal foot).

Paint the walls this soft neutral - bm nimbus - http://www.benjaminmoore.com/en-us/paint-color/nimbus

Now - Instead of trying to have the mirror by itself visually fill the space above, lets get you a gallery for the large upper wall. The mirror is not the best choice if what it reflects isn't something outside. Set aside an art budget - this is more important than the slate for now. First, look for an interesting table top sculpture to be the only thing on the top of the mantel shelf - it should be about 2/3 as wide as the firebox opening at minimum for scale. It could be as large as 2/3 of the width of the stone.

I'll bring you back other links for the gallery later on if you test these colors and want more help.

I generally find it useful to empty the shelves before I paint.

The simplest fix would be to replace the current shelf with shelf equal to the thickness of the mantel (horizontal stone). Then, I would place two legs perpendicular to the mantel legs that are match the thickness of the new shelf. You could duplicate those legs and position them flush with the corners of the fireplace wall. Of course, you can add a shelf in the middle of each of the new niches. Again, I would make these shelves as thick as the top shelf. Hence, I would not have two shelves like you have currently in each niche.

If you use wood to construct the legs and shelves, then I would keep the main shelf flush with the mantel since I think that the overhang from the upper shelf would create a fire hazard. However, I could be wrong.

You don't have to fill the whole mantel.

hi groveraxle, I was not using a real graphics program. I used the paint program that comes with Windows and added the details so that the OP could have an idea of how changing the scale of the shelf, etc would make a big difference. I should look into adobe elements or the like.

to the OP, I like the stone and would not paint it.