Try the opposite of what are you doing:do a very thight design (like the opposite of stretching an image)and do only one side and mirror it and see what it happens
Edit:like this
ORIGINAL
TIGHTENED
Maybe that will stretch the pattern perfectly and if you have the nerves do like a perfect matchup so it could look good mirrored
You know that the same happens to the long texture walls.If you want to make it look good you have to do a perfect loopable texture so you won't notice where it starts and ends
