I haven't read all the responses in detail but I have been in the same shoes and here are some suggestions and things WE did that helped immensely.
First - just realize that this is a HUGE transition for your daughter. Like any transition, she needs your help to navigate all the emotions she is having etc. And it is PERFECTLY NATURAL for her to feel love/hatred for the new baby. There's a great book "Siblings without Rivalry" and in the beginning, he has you imagine your spouse/partner is coming home with a new wife/husband/spouse - and you'res supposed to feel happy about it. Pretty funny....
Anyway, with my DD we used the Playful Parenting approach. We had a real "b*tch session" about the baby together where I tried to help my DD put her feelings into words. "I want the baby to go away. I want Mommy all to myself. It's not fair that Mommy can't play with me right now." etc. It seemed to help a little bit that DD realized that I understood her feelings. And she was able to release some of those "bad" feelings so her good feelings could come forward.
Next - we used her favorite Gymbo doll to act out all the bad things she wanted to do. "Don't hit the baby Gymbo" (with a baby doll) - "you bad Gymbo" I would yell and she just died with laughter. It was a release for her to see someone else getting "in trouble".
Also - to help us both reconnect, I got out all the old pictures of her as a baby. I talked to her about what it was like when she was a baby. It made me very nostalgic and I felt alot more "charitable" towards her afterwards.
And for yourself - let go of guilt. Some of what you are feeling is hormones talking ("ugg" says cavewoman "baby first-ugg"). And some is plain ole mommy guilt. Try to think about what kinds of things YOU liked to do with your mom as a child. And try to cultivate that with your older DD. You don't have to spend every second with the older child for them to remember the "good times".
A big part of helping your DD get over the sibling rivalry is to make sure that you both focus on YOUR RELATIONSHIP as a separate entity from the baby. Don't make the baby the excuse for why you can't do something....instead of saying "I'm feeding the baby" for example, just say "mommy is busy right now and I will help you in 5 minutes when I'm done" - just like you were doing laundry or something.
I also took as much advantage as I could those first few months when the baby sleeps alot to make sure my attention was focused on my older DD. It wasn't even doing anything specific - but just sometimes making silly faces with her and really doing that connection thing where you look her in the eye. The house looked awful and nothing got done - but my DD is now (6 months later) confident that Mommy still loves her and that even though things have changed, I still WANT to be with her and have her around.
Anyway, those are things that have helped a bunch in our relationship. Maybe some of them could work for you.
I know it is so hard! Good luck,