This is completely a lay opinion but I dont think its uncommon to find it easier to bond with second children more immediately than the first. I felt it with mine, I've seen it with friends, I am watching it with my sister at the moment. I am not saying most people dont bond with their first, I just think it often comes more quickly and more easily the second time.
All I can think is that with the first, the shock of having a baby, and having to look after the baby all day every day, often having breastfeeding issues, the lack of sleep etc can be pretty overwhelming. I think usually by child two, you have a handle on parenting and you are almost 'primed' to love another one straight away.
What you went through with your first birth is an added and very specific and significant layer of trauma.
You have bonded with your second child so clearly you can bond with children, and you want to feel more bonded to your DD1 so that is all positive. I think the point that it is not too late even now is a good one. You may never get that feeling that she is 'your little baby' (- I am so guilty of babying my second child in a way I didnt do with DD1) but you can get to know her and bond with her for who she is now.
When you say she is high maintenance do you mean 'attention seeking' and if you do, do you think she is looking for your attention and affection because she subconciously feels the distance? Is she jealous of her sister? Can you think about setting up some quailty time with her doing something you both like so you build up your feelings for her?
I think the older child often suffers from our unrealistic expectations of their emotional maturity- I expected a lot more of DD1 at 2 and a half than I now do of her sister. At some basic level I am better at parenting DD2 because I have had more practise at parenting, I am more tolerant with her generally than I was with DD1 at a comparable age.
I suspect talking to someone would help, but I am afraid I dont know who to suggest. Just talking about it at all it probably a very positive step.