So my ex lives with me and I talked him into getting a puppy because I thought it would help with his depression, anxiety, and loneliness. (Spoiler alert - it did help with all these things.) She's solidly his dog, when he moves out (please God let it be soon) the dog will go with him.

I take the dog to the dog park probably 50% of the time, I feed the dog probably 50% of the time, and she's sleeps in my bed with me probably 30% of the time. I take her to most vet visits because of my flexible schedule and also help drop her off and pick her up from doggie daycare.

Why do I do all of this? Because it doesn't matter who technically "owns" the dog, the dog has needs and must be cared for. If his work schedule makes it difficult to take care of her as well as she needs, then I'll step in and help because I love this dog whether she is "mine" or not.

I'm not going to lie I find it very strange that she won't help with your dog just because it's not "her" dog. I suppose it's legally property but it's also a sentient creature with an emotional inner life that loves her. It's very odd to me.

Does she perhaps have a side of the story you aren't sharing? Does she take a much larger share of the cooking and household tasks, leaving her with less time time for the dog? Does she work very long hours? Is it possible she really doesn't even like dogs but didn't tell you the truth because she didn't want to disappoint you?