Hi A.! First of all, I just want to congratulate you and your husband on your blessing! What a wonderful story!
And second, I literally just went through this with my daughter, and she's finally sleeping through the night at 6 mos. I second the previous poster who recommended Dr. Weisbluth's book. It's not the easiest read (mind-numbingly boring at times) but the man is a genius with babies and sleep. So pick up a copy and read that.
My guess is that your daughter hasn't yet learned how to self-soothe, which was my daughter's problem. I actually had to train her to self-soothe, using a pacifier (she only likes it in the crib) a small blanket and a stuffed Mickey Mouse. I started cuddling her with certain of these items that I noticed she seemed to be drawn to -- for example, when she went to bed with me (yes, I did the same thing for sleepless nights!) I noticed she would grab a peice of the sheet and put it up next to her face, and burrow her face in it. So I took a crocheted baby blanket (has "holes" so I don't worry about her smothering) and started putting that next to her face when she would cuddle up for sleep with me. Next I noticed that she loved to hold and touch her Mickey Mouse toy, so I started bringing that to bed with us, too. Now, as soon as I put the blanket near her face and give her Mickey, she closes her eyes and goes to sleep (sometimes it's still a struggle -- dont' get me wrong!). In the middle of the night, if she wakes up and fusses, she will now grab her blankie and her mickey mouse, and go back to sleep. It took three months for this to work, but eventually it did. She has been sleeping through the night for a week now (knock on wood!).
Every baby is different, and you'll have to try and figure out what your daughter's sleep issue is. My son slept through the night from age 3 mo, and learned how to self-soothe himself, so I never had to deal with it like I did my daughter. But you'll figure it out, and soon you'll get that sleep you need! Good luck!