Begin with a chisel to take it down and start over. Sorry, hon, what EXACTLY were you after with this ? It isn't even safe or to code with the mantel support....which is supposed to be what?

I would start over. Sorry, but it looks like a heck.
First: there is a minimum space requirement between the firebox opening and any flammable material. Safety must be considered when designing a fireplace surround.
Second: make a plan in advance. Consider what materials will be appropriate and attractive for your project. Take accurate measurements.
With a resource like Houzz available for free, there is no end to good ideas to inspire you. There are also instructions for many basic projects.

Is that porcelain tile on top. If so you could continue on around the firebox. I like it.

Are the corbels too long? I am trying to support my mantel. I initially wanted the mantle floating. But it’s way too heavy. Fortunately, I seldom use the fireplace. The whole project came about after unsuccessfully sheet rocking over a TV Cubby above the fireplace. Next, I need to tile around the fireplace....in the picture you can see cement board around it now. The original tile (done by house builder) was attached partially to sheet rock, which seems crazy to me.

You will need toremove the mantel to the tile work anyway. Corbels are decorative pieces made to hold up mantels or counter overhangs but they are never like what you have. The tiling should have satrted with the fireplace surround. There is no problem with tile on drywall with a gas fireplace.

For an affordable way to get a unique and elegant look,
check out these tile options that are easy to install https://www.houzz.com/projects/1509556/wow-tiles-bold-unique-elegant