You will do fine! Babies sleep a lot anyway so your daughter will get plenty of attention. I was 20 when I had my daughter, she was 2 and a half when my son was born. I was worried to but really haven't had any problems, I think the most difficult thing was going to the shop and getting them both in and out the car! I'm now 30 wks with my 3rd, my son will be 2yrs and 2months old when this baby is born. My daughter starts school in a few weeks so its going to be a struggle at first with a newborn and a toddler doing school runs but its all about routine. Just try and get in one asap and all will be ok :)
Im 22 and have a 3 year old daughter, a 4 year old step daughter, and am about a week away from delivering my son. I am also nervous because both my girls are handfuls. Its scary to have to take care of a newborn and two toddlers at the same time. My strategy is to get tge girls as excited as possible about yheir little brothers arivel. I feel like if they are excited and I try and get them envolved in helping with their little brother they will be more accepting and excited. (They are little girls after all). I think its important to still keep them involved in the whole experince of having a new baby around. I plan on letting them help me make bottles and picking out his outfits and stuff so they still feel like they are being big girls but spending time with mommy as well.
I have the same problem except my son is 1yrs old. The only thing I csn tell you is show your baby all the attention now, that way when the new baby gets here she wont feel left out. At the end of the day we're moms of 2 and we'll love both babies equally