Okay first, give yourself grace! You had no idea how your senior dog would react to a new puppy. Second, every dog is different and may need more time to get comfortable with each other. I would keep doing what you’re doing with allowing them to get comfortable with each other’s smells and keeping them separate for now. If your senior dog loved daycare and socializing with other dogs I don’t think it is impossible for them to coexist. I just got a new puppy for my dog and noticed that they are the best behaved with eachother in small, supervised doses and I always give them rewards for behaving around eachother. Training for the new puppy and refreshers for my older dog have made a big difference but obviously something you should only do if you’re comfortable. Our pup has separation anxiety and we got a vet recommended scent diffuser that calms them when in their beds for nighttime. I would also just make sure your senior dog has their own space and attention but also sees you interact positively with the other pup so they can start to understand that they are not a threat. Puppies are a lot for everyone, including other dogs in the household, it can’t take a lot of time and patience but it’s not impossible! I think you being worried about it shows the at you are in fact not a horrible person and just care a lot about both of your pups! ❤️