Fantasy vs. reality
I'm off the clock and this is not professional advice, but I'm a psychotherapist and, based only on the information you've given, I'd agree that this is something that will likely pass. Like BubbaTD said, there's often a "let down" after we have anticipated something for a long time -- newlyweds go through it, recent grads go through it, new mommies go through it. I think part of it is that the reality of something, no matter how wonderful, is never quite as amazing as the fantasy you have about it.
I think what has to happen for a lot of people is a meeting point between what you had fantasized about and what the reality is. I read a study recently about how mothers who give birth to a child with Down's Syndrome have to take time to re-think their future and their hopes and aspirations for their child. Those mothers who can accept and deal with reality (rather than get stuck in mourning the fantasy and the hopes) fare much better, are less depressed, etc.
"Fantasy" puppies are all snuggles and cuddles! Reality puppies are poop and whines and sleepless nights. The sooner you can refigure your emotions to match your reality instead of your fantasy, the better you will feel, I think.
I'll add that I also went through this when I first got my Milo. I had erroneous beliefs about what a dog would provide me -- and basically thought I could get a living teddy bear who would dispense affection at my every whim. Oh sure, I'd read about dog training and about horror stories of ill-behaved dogs, but I would never have that dog. My fantasy dog was perfect! Didn't smell, didn't cry, didn't lunge.
What I got was a puppy (too young separated from mom and littermates) who wanted to bite me all the time, who kept me up all night, and who ate away my savings in vet bills. I couldn't get him to mind me. I couldn't figure out what he wanted. I cried and cried. Here all I wanted was a puppy to cuddle -- and what I got was a shrieking, biting terror! I took his puppy-ness personally, if that's possible.
The better I got with communicating my desires to Milo (training, etc.), the better I felt. We really did build a bond and a relationship that amazes me to this day. He is the best companion I could ever hope for, and I can't imagine life without him! Is he a teddy bear to dispense affection? Nope. He's a dog. But he's a darn good dog.
You'll get there. I'm about to bring a second puppy into our home, and I feel much better equipped this time. I know what to expect with a puppy, and I know not to take it personally. I also know that puppyhood passes, and the more I can do for the dog during puppyhood, the better of a dog he will be. On the other hand, don't be too hard on yourself for not already achieving perfection. Dogs will teach us patience. They need time and repetition. It'll all come together, I promise.
If you think you're alone in this, go on petfinder.com and look at all of the 10- to 15-month-old puppies who need new homes. Lots of people can't deal with the reality of puppies. It's much more fun to fantasize.