Having a baby is a major life event! It isn't easy to add another person into your relationship with your husband. This is only 3 weeks new! You have the rest of your lives. Take time for you two together. Even if it is little things, like renting a movie and putting the baby down in another room so you can cuddle and snuggle. Or how about Grandma watches the baby and you two go out to dinner and do something just the two of you.
Sounds like it was very honeymoonish when you brought the baby home and now that the reality has set in... it is real, average, everyday. Life turns out like that. It won't all be easy - but it shouldn't be HARD either. If you are feeling very, very down, you should talk to your doctor about it. The post partum period can be an emotional roller coaster. You've just had a miracle, people are always asking about the baby, wondering how she is, you don't have anymore time alone, you are 'mom' 24/7 and everyone made it look easier than it turns out to be.
Be gentle with yourself and your husband. You two will find your way.
Congrats on your beautiful daughter!