My heart breaks for you. But don't feel guilty because you cant help him, I think you done so much for him already by giving him a chance. Some dogs have what I call "brain ghosts" and as much as we want to, we or they can not change their behavior.
Summer before last I took in an adult Golden mix found walking the streets. Could not understand why no one was looking for her, so sweet and well behaved. Well after a couple of weeks she started showing her true self, sever anxiety [not separation], would have "attacks" out of the blue, no triggers. After 3 months of trying every thing and resorting to meds, she still tried to go through a window,a neighbor saw her and called me, she had chewed up my wooden window trim, made a big enough hole in the glass to stick her muzzle through it, blood all over the place. I had to make the hard decision to set her free.
Sometimes we have to let them go because we love them and we have the power to set them free from their pain be it physical or mental.