I think quite normal - certainly most people I've spoken to seem to have been through it.
Best bit of advice I've been given? From a mum of 4, 'you are giving them so much more than you are taking away from them'. And having seen the 2 of them together, I genuinely believe that. DD1 and 2 are 25 months apart, now 4 & 2 and, whilst there are the usual sibling rivalries and fights, when they get on well, it's just the loveliest thing to see. They're real little playmates and are sharing a bedroom through choice. DD2 wasn't much of a 'threat' to DD1 in the first year, babies do a lot of sleeping when the toddler's about, so DD1 had had more than enough time to get used to her before she really started to make her presence felt. Now, she doesn't remember a time before DD2 was there.
My other thought is how much I value my relationship with my own sibs - 1 of each - there were a good few years in the teens when we didn't get on, but we're all really close now.
I think there's an element of pg hormones that come into play at the end too - am currently 41 +1 with DC3 and DESPERATE for it to turn up and yet found myself having a nap this afternoon and feeling really quite tearful about the fact that I wasn't playing in the garden with my girls (GPs are staying, so they're more than happy), and what was I doing having another when I should be looking after them.
Oh, and I agree with all the comments above re bonding with 2nd+ child - I keep being asked if I have any inkling about what sex DC3 is - really haven't had a chance to think about it this time round!
Just make sure you get time for cuddles with your DD and enjoy both of them - tis hard work at the beginning but well worth it and lovely to watch their relationship grow.
Good luck!