Flooring Tile - Shower tile edges

I'm about to gut our guest bath and will be retiling the shower. The thing I am worried about is were the tile stops going up the wall and it transitions to drywall. There are currently bullnose tiles there that curve very nicley and make a nice transition. The problem is in the corners where one wall meets another at right angle. There currently is a little corner piece that looks like maybe it was cut out of a bullnose tile. This seems like a difficult thing to make and unless it was done just right, wouldn't look real good.

I didn't see these corner pieces at the Big Orange or Blue Box. Are they something you generally buy or have to make? I also thought of using some extruded plastic molding around the top instead of bullnose. Any problem with that? Any other trick to making this corner? Thanks.

Why can't you simply 45 those pieces to make a nice 90 degree inside corner? I must be missing something.:)

Thanks. That is a much more obvious solution. That what you get for assuming the way it was done before is the "best" or "right" way to do it.