*nods head*
Mastiffs are considered "puppies" until they are 2. I guess I've been holding out hope for her...
My Pup Pup was just like your Sofia. Extremely dominant. Her sister, Blaze, was very dominant as well.
They ended up at 8 weeks old getting into a very bad fight--leaving Pup Pup with a nasty scar on her ear that she had the rest of her life. And this was at 8 weeks old! I couldn't imagine breaking them up at 100+ lbs! I found a new home for Blaze and she did fantastic.
What I'm trying to say is that, I've been in your shoes--so my words are not coming from judgement but rather, experience. I wish you success going forward.