HI J., to me, the point is not to say something to make it all better for your daughter, the point is to help her slowly adjust to the new addition in your lives. Put yourself in her place, remembering what it was like to be five, and imagine what it would be like if you only lived with one parent, and now it feels like that parent's love might be taken away by someone else. Try to overlook how annoying her tantrums are, and instead think of them as her way of expressing her fears and frustrations. It is always hard to adjust to a big change, especially for a five year old who doesn't have the maturity or patience of an adult. Your situation would be hard for a child of any age, even a teenager, but it is especially hard for someone so young. I think it would help to bring your sweetie into her life very slowly. You and he may be discussing getting married in two years, but she does not have to be included in that discussion until the time comes. In the beginning, only include him in activities outside of the home that she enjoys, and for short periods of time, like walks in the park or going out for lunch or a movie. After she starts to bond with him, include him in more activities in your home, and for longer periods of time. Don't hold hands or have physical contact with him in front of her other than what you normally have with your other friends. At the same time, make sure that you have plenty of quality time alone with her, so that she sees that him coming into your life will not mean her being squeezed out. Read to her while cuddling her, play with her on her level, and make sure she knows she will always be your special girl no matter what. Her security in your love will help her feel less threatened by any life changes such as a new sweetie in your life.
About her tantrums, even though they are understandable, they are unacceptable behavior. Since she has developed a pattern of tantrumming, it is safe to assume she will do it again sometime in the future. This gives you an opportunity to provide her with a valuable lesson about consequences. Sit down quietly with her and tell her firmly that her behavior when she screams, kicks, etc is unacceptable, and that the next time she does it, she will have a consequence. Tell her what the consequence will be and make it involve taking away something that is important to her, such as TV time, or taking away a favorite toy for a day. Make sure the consequence involves losing a privilege instead of adding a punishment. Then, if she tantrums, take away the privilege, telling her that you are sorry she chose the consequence. Make sure she understands that she will get her privilege back if she does not tantrum for the next day, and that after that if she tantrums again, she will lose the privilege for a day again. Be quiet and firm. She will hear you much better that way than if you yell (because yelling causes people to tune out.) You must be firm and don't cave in or be wishy-washy. Just be quiet and firm. It may take a couple of times for her to really get it, but when she sees that she can choose her own consequences, and that there is no reward for tantrumming, she will stop doing it.
About the "good whooping." I don't know what that means but I hope it does not mean that you are hitting your child. Hitting will only cause more problems between you and your child. If you want to be respected, act like you deserve it. Acting like a tantrumming adult will only teach your child to continue tantrumming. If you are unable to stop hitting or yelling at her because you are so frustrated or because that was how you were taught that parents act when you were a kid, maybe it would help to get some counseling. There are more effective ways of parenting, and you can learn to guide and discipline your child in ways that will better work toward improving your relationship with your child and increasing your own self-esteem rather than further destroying them.
Good luck and let us know what happens. It's clear that you love your daughter very much from the energy that you have put into this issue, and I wish you the best of luck.