I missed it the first time but I think that attaching a kennel to the door to potty the dog more safely and maybe with no leash is a great idea. You could even put a tarp over the top so he could not climb out if he were inclined to try that in a panic.
For what it's worth I recently took on a foster shepherd that was abandoned at my vet and I too went through a phase of oh no, what have I done. We got through it and I did what I set out to do with that dog. Hang in there.