I think your feelings are probably normal, but there is hope!
I think there are two ways to handle this, first that you need to get things under control with your daughter, and second that there are things you can do to help her prepare and handle the birth of a new sibling.
Before your new baby is born, you probably need to set some ground rules and limits for your daughter. It may be that you have already done this. The most important part about this is that you have to be consistent and stick to your word. Your 'yes' means 'yes' and your 'no' means 'no.' The rules and limitations are the same all the time. She should never get her way by throwing a tantrum or whining. That only reinforces the behavior and tells her that her bad behavior is working. You can model for her how to use her big girl voice and how to talk politely. Let her learn that this is way more effective than throwing a fit.
Although, sometimes kids don't understand why they can't have something even when they ask nicely. In these cases, you can focus on what she can have instead, rather than what she can't. For example, "I can't let you have candy at the store, but we can have a snack in the car. Would you like a granola bar or fruit?" Giving her more choices (like 2 at a time) can help her feel like she has a little more control over her world, but it's not an overwhelming amount to her. When there are no limits being enforced, kids feel overwhelmed by their environment, so limits help them feel safe. Having a couple of choices to choose from many times throughout the day can help her gain self confidence and feel some self control (Would you like to wear these pants, or these? Would you like milk or water with lunch? Would you like to help me wash the fruit or the vegetables? Would you like to clean up by yourself or do you want me to help you?) This can help a lot with strong willed children.
Ok, now for the new sibling thing, there are lots of things you guys can do to prepare: visit the hospital and look at the new babies, spend some time with a friend who has a new little one, play with her baby dolls, talk about the new baby a lot and explain things to her in ways she can understand. When the baby arrives, expect that she will be a little out of sorts for some time. She may start to act more like a baby herself, do things she wouldn't normally do, etc. Just remember that this is new for her and she may not know how to handle such a change. If you can, spend some one-on-one time with her each day, maybe while the baby sleeps. Let her know that she is still so important to you and how much you will always love her (this does not mean that you should be permissive and let her do things that are normally off limits, as this would add to all the 'changes' and could confuse her. Be sure to keep your boundaries the same and consistent). Let her help with the new baby and be involved if she wants to be. It's also important to keep her routine and schedule very consistent and close to how it was before the new baby, or as close as possible. Included in this would be that if she went to child care, still have her go a couple of times a week so she's not totally thrown off. The more consistency, the better.
I guarantee she will not resent you or stay mad at you. It is a change for all of you and you will all eventually adjust. I've never heard of a child who permanently resented their parents for having another child, so stay positive and things will settle in time.
Ok, sorry this is so long, but I hope it helps a little. best wishes!