Holy Moly! I think you may need professtional help on this one...wish I could help.

Beth .......if you are out there please give some advice!

What waterproofing system is under that?

https://www.ceramictilefoundation.org/homeowners-guide-to-hiring-qualified-tile-installer

Hire a Pro. Now. Before you rot our your home from moisture leaking into it. That all has to be ripped out and redone properly. With the correct waterproofing. And with some math and thought, and a wet saw with an experienced operator.

This is a mess that now needs to be rippped out and done over and I too am wondering what you did for waterproofing before starting the tile.

The waterproofing question will need to be answered for any options other than rip it all out.

Don't rip out a thing. Have a solid surface fabricator build a waterproof custom insert.

I'm sorry,,,i have no save for this niche. grout is not waterproof. niches are very difficult to get right, especially if this is your first tile rodeo.

your bottom tiles inside the niche need to be slanted, slightly, down towards the shower. if they are flat in there, it will hold onto water. water has to be able to drain out. so those need to come out and get redone w/a slight slant.

I mean, you might be able to dig out that mortar and add some thin pencil liners, but I don't know,,,it's still not going to look great. they used thicker liners w/this niche

this is a simple niche.notice how the tiles above and below are perfectly flush to the box. no thin or short cuts. you have to figure your layout first, on paper or on the floor, with a lot of math, before you even begin to tile.

you need bullnose tiles for all the edges, (see the ones above how the glaze extends onto the curved edging? that's what you want for all you borders and edges) you purchase them along w/your other tile. all those gaps, unsealed tile edges, even your grout lines, allow water to absorb in and get under the tile. I have no idea how you even water proofed this, but I can tell you,,,,it's a tear out. sorry.

go to John Bridge tile forum and do your research,,,or hire a pro.

many ways to tile a niche,,

this one, below, isn't even a pro tile job,,,but for a beginner, it would pass.

So sorry Sydney that your first tile project didn’t work out as planned...I hope a tile professional can come in and maybe just tear out the niche and add the proper bullnose that is needed to finish the edges properly..as long as the shower was built properly..waterproofing and such...good luck...hope to hear from you with a happy result!

Kudos for the bravery and the will to do this on your own! But you should hire a pro to make this right. You could ask at your local tile shop to recommend you a contractor for this. We have a great communication with some of the best contractors in NY and could always recommend. So probably your local showroom can do the same, especially if you bought the tile there.

I take back what I said about not ripping out a thing. Rip out the center divider, then have a solid surface shop make a custom insert fabricated with an outside flange. Silicone the flange to the tile and forget about it: it's now waterproof. It will look like you planned it that way.

taco shy of a combo plate

@Joe, I get what your thinking. Until the waterproofing questioned is answered your replacement niche may be the only thing that actually is waterproof. We just don't know how to advise to tie into existing waterproofing IF there is any. It could be green board for all we know. Without Sydney responding this thread has hit a wall.

another person shuddering at DIY for a *shower*, the biggest cause of water damage in most houses.

but as to the niche, you have to plan ahead for the edges -- either your tile needs to come with a bullnose, which can be used at the edges, or you need to buy edging for this kind of corner, which is inserted before the last row of tile is laid, with parts that extend into the thinset underneath in each direction. you'd have to pull out a lot to put that in now.