I don't know if you still want more help with this, but I'll post anyway, since I solved it a different way than Sarkut.
It's actually your back wall that was the problem. Here is a rough image to illustrate:

It is a bit of a sloppy rush job, but it hopefully shows the point. When your viewpoint is downwards like that, surface edges that are normally parallel straight up and down will instead need to converge to a point below the image.
Start by getting the side walls matched up with the floor using rotate/zoom, then get the back wall matched up with the sides. That should work. Good luck!
