hello. I ahd my dd when ds was 19 months old.
It was very hard for about 2 months, but then things just 'clicked', through no action on my part, just everyone getting used to having a new person around.
My biggest tip is to get anyone who can help (friends, family, neighbour) with the oldest as they cannot really help with teh new baby. Ask them to take out the eldest for walks, playgroups, anything to get her out/ exercised/ interacting. My friends woudl group togather and take my ds along with them when they went to a playgroup so there was strength in numbers. That way I could at least nap for 20 minutes or so and have some new baby time.
If you can afford it, also ask friends nannies/ au pairs/ whatever if they can 'have' your dd while their charges are at school. You will have to pay them but not much as they will have 'down' time then. Again, it's only an hour or two but will make a huge difference especially if you can do it a few times a week. I was lucky enough to have this kind of 'mother's help' for 2 weeks and it transformed my life
Students are also a great source of reliable, mature play time with your dd. Even if it is still within the house. Maybe your neighbours kids are due back from college/ uni break for christmas and are keen to earn a couple of pounds?
above all, I promise it does get a LOT easier. and it's perfect timing as you'll have help around at christmas. just make sure you use teh time to slepp.
oh, and most of all, if you can - express milk so you can sleep while dh feeds baby, preferably a night feed.