A threshold is always need when you are dealing with different heights; at this point, you need to cut/remove tiles to accommodate the width of a marble threshold. Another option is to look at Schluter trim pieces, if you're looking for a more contemporary look. But, they still need to go under the tiles; so a row of tiles will have to be removed.

I am sure there are other saddle type trims to blend the height differences better than this. Ask some flooring experts in your area or search here on houzz.
Your new tile is beautiful but this could have been addressed better at the time it was put in.

Unfortunately it was put in without my knowledge a couple of days ago, now I need to figure out how to fix it