I've used B12 paste, it didn't work on my horse at all, but have friends who use it and it works for them.
I think BnP is right maybe try using rescue remedy for yourself, as a lot of it will be you, esp if your waiting for it to happen.
I think all banned substances are banned across the board, but if you just take him out to a few outings, don't compete see if they help and perhaps that will help you both settle, then you can try entering once you are confident in yourself and him.
Its hard to get hung up on bad experiences, just aim for small things, and once you've smashed those move up. nothing wrong with baby steps. And each set back is ok, its how we learn.