Hi Anne
Welcome to COL.
I have a black lab, Sapphi, a golden retriever Max and cocker puppy Barney arrives in 9 days!
I'm afraid I'd agree with Colin, but it has to be your decision.
Every dog is different. Sapphi was like Millie, a smartypants at puppy class and so obedient until .... adolescence. She was always a lively puppy and always on the go but as I said obedient, quick to learn and walked beautifully on the lead. In adolenscence she was still very lively but less willing to do as asked and pulled so badly on the lead. She was a little trying shall we say!
I waited until she was almost two before I felt I could get another dog.
Max however is a totally different kettle of fish. He has always been laid back and really calm even as a young puppy. Don't get me wrong, he gets excited when playing with Sapphi and the other puppies but he's certainly not over the top. He did well a puppy class but certainly wasn't the class smartypants. He steadily got better and better at obedience.
I took advice from my trainer who knows both Sapphi and Max about the right time to get another puppy, (which will be the last!
). She agreed that Max was mature enough now but there would have been no way I'd have got another when Sapphi was just 1. That would have been a nightmare.
So, as I said every dog is different. I hope it does all work out for you, and I'd love to see pictures of Millie.
Good luck and keep us posted.