I am so sorry you are struggling with this, it is really hard b/c it doesn't fit the picture you were hoping to have of loving siblings and/or if all of your friends siblings love babies and had no trouble adjusting.
My oldest was a similar age and I think in some ways the older they are, the harder it is for them b/c they are used to having you all to themselves and then the little "invader" comes and takes you (the most important thing in the world to them) away.
There is a book "Siblings without Rivalry" that I thought was really really helpful in how to assist the oldest. It gave me a much better understanding of how they really suffer and what it is like for them.
For us, there was no magic wand. It just took time (not the answer you wanted to hear I know). We tried lots of positive reinforcement for big kid behaviors, for kind behaviors, the present idea, alone time etc, but it was a tough transition (and still is on some days).
I just tried to make sure that they were never alone together and to reinforce the message that it was ok not to like the baby (and honor the negative feelings) but to try to reinforce the message that hurting is not ok.
My husband also used it as a time to connect and bond a little more with the eldest and take her out of the house and do 'big kid' projects/adventures when he was around. I think sometimes the reduction in stimulation (being away from baby/crying etc) was helpful too.
We also tried to make sure that oldest got lots of attention from visitng friends/family/strangers who all want to talk about how cute the baby is, but often ignore the special big brother.
We also tried a special mom/oldest date of their choice once a week (away from baby-use dad or babysitter to watch youngest) if the oldest earned enough tokens (from kind hands - catching them doing gentle behaviors or chosing not to hurt).
Hope that knowing you aren't alone helps, but I think your best bet it to start w/ the book. Good luck!!!!