I agree with Deb and CeeCee, this dog needs patience and a continuity of training. And no mastiff will react well to being snuck up on whether tired or not.
I believe you are in a rush, a real rush to perfect your dog. You admit, you get worked up...she feeds off your energy which isn't good...You don't care if it's a phase...and that she'll grow out of it...that she's a puppy...It's not going to fix it today. Do you hear the lack of respect that you have for the process, the natural phases that these animals go through. You're ready to nuke her till she glows.
Tell you what, put the shock collar on yourself and experiment with all the shock levels on yourself...get the wife to do it to, and when you both have a clear understanding of the the pain you are willing to inflict, then decide what's best for your situation. I'm not "flaming you", I'm trying to give you perspective.
IMO you will never get a calm, confident dog that is perfect, using negative and pain driven techniques. You might get an animal that will lash out at you, and whose fault will that be? It's all in what you are willing to invest: time, training, affection and understanding.
Stella is a three dimensional creature, she has needs not just wants. Wants can wait, needs can't. And, by the way, the old adage applies, "You get more with a carrot than a stick." Dogs respond to praise, food and respect/love.
PS. I still thank you for doing the toughest job in the world. In my book you are still a hero. Be a hero to your dog. Take your time, you are creating a masterpiece.