Oh I am so sorry to hear that you're having to return your pup. I can only imagine that this is one of the hardest decisions you've ever had to make. I don't know you're whole situation so it's hard to give advice (not to mention, I'm dealing with my own issues of semi-aggressive doggie) but I just wanted to say I'm sorry and hang in there. Maybe now with the knowledge you have from the trainers, etc. when you get your new dog, you can start off right, right off the hop and that may deter the aggressive behaviour the second time around.
Good luck!
__________________  Jillian  & Maxwell  & Ruby  |