When we got our (now five yo) dog she was 8 weeks old, and I remember it being the Easter holidays, DH was away, and it rained solidly for several days. We couldn't take her for walks as she hadn't been vaccinated.
I was amazed to realize that the way I felt was very similar to when I had PND with my babies. Awful.
This was the lowest point, and it did get better once we could take her out, and once I started puppy classes and worked on the training a bit.
She always was a beautifully natured dog, just more lively than I expected, plus I hadn't been the one to push for dog ownership, that was DH, which I suppose made me resentful when I found myself staying in with her, or clearing up poo from behind the sofa.
I also remember the trapped feeling, and even now, as an SAHM, I never leave her for more than about 4 hours, but feel bad about that. DH often works at home, so it's only about once a week on average, but it's still a pain in the arse to have that feeling of guilt on leaving the house.
As I said, I didn't really want a dog, so I kind of felt justified in the negative feelings (although I adore her now and am as kind as kind could be to her), but if you're feeling like this and you were the one pushing for a dog I think, as you say, you didn't really think it through and maybe it's best to do as others have said and re-home it.
It will get better, but the situation remains the same; you are responsible for this creature and it never stops having a bearing on what you can or cannot do.