Old dog and new dog play struggles- need advice

We recently adopted an 11 month old puppy from the shelter 5 days ago. She has been a pretty good dog, but of course needs to get adjusted to her new home and having rules.

My concern is my older dog, Shiner. We wanted to adopt for a while now, and unfortunately had to put my 12 year old dog, Tucker, down a couple of weeks ago due to his rapidly declining health. It was hard on all of us, including Shiner. As I mentioned, we wanted to adopt, even before we knew we’d have to let Tucker go. We felt ready because our hearts are always open for dogs and a big reason too was that I could tell Shiner was confused and sad. I thought a pup would help her.

Shiner is a submissive dog and Tucker was always the alpha. We bring in our new dog and they do fine together and do normal dog greetings, everything is fine until our pup wants to play. She likes to bite Shiner on the snout and ears and Shiner tries to move away from the new pup, but she follows and continues until we break it up. We break it up because Shiner clearly doesn’t want to play like that, but she does nothing to stand up for herself. Tonight it was a little worse. We let our new pup try and play with Shiner and she even started playing back with her, but then quickly let the pup control things again and we heard some whimpers from Shiner, so we once again broke it up. I find myself upset that Shiner, who is a Great Pyrenees/Anatolian mix, won’t stick up for herself, against a dog she has a good 75 pounds on. I’m sure that doesn’t matter- but in the moment I really just want her to stand her ground.

I am looking for suggestions on what we should do moving forward. We’ve had amazingly calm dogs with Tucker and Shiner, both were super easy to train and very good with each other. I feel guilty now for thinking Shiner needed a friend so soon after losing Tucker. I want to make things work with the new dog and I know it hasn’t even been a week, but we aren’t sure what to do for Shiner’s sake. Please let me know your thoughts.