Mine is the same way because i used 3 4 thick tile on my wall.
Tiling uneven walls shower.
With the uneven ceiling level you would normally level or laser a horizontal line that runs around the shower enclosure at the top of the tile if leaving a top border tile free see below then measure your row lines down from there to the bottom and start with any bottom trip and cuts and actually place the tile from bottom up to the top.
Attach a straight board to the backer board along the line.
If any part of that should get wet you could have some issues.
You could have wrapped around the edge w the tiles for a neater look.
Draw a leveled line where the bottom of the second row will fall.
Begin by removing the old tile or the fiberglass shower wall.
Whatever you put there you are going to see the uneven tiles.
This is a dusty job so use plastic to cover your doorways wear a respirator that s rated for dust cover the floor with a drop cloth and make sure there s plenty of ventilation.
I ended up going w a 1 thick basalt chair rail.
Also tile is heavy so put down blankets to protect the floors and tub from being damaged by falling pieces.
When you ended the tiles at the window why weren t they all cut evenly.
If the shower base is uneven tiles will have to be cut to fit the last row.
Find a matching thick chair rail edging 12 piece and you ll be fine.
This will provide a straight edge for the tile to follow and help support it while it dries.
You should have used the proper substrate for your shower walls.
Just get some chair rail molding or some type of 1 trim pieces.