My wife and I have a daughter age 14. We love her very much, but we made many mistakes. From the time she turned three she began having hours long tantrums and screaming fits. Time outs were futile, so we ended up giving in to all her demands. Now we have a unhappy home. Our daughter has run our home for years and ran our joy away. Our marriage has been all but non-existant. I seek hope and advice.
So you are telling me that you and your wife are so weak-willed that a child can get whatever she wants out of you. God made you parents for a reason and so far you are failing your job. The solution is simple, you and your wife determine that you will only do what you think is correct and reasonable. What your daughter wants no longer matters.
If she pulls a tantrum, you pretend that she doesn't exist. You go on about your business. Most likely she won't believe that she has lost control, so she will try to outlast you, but realize that a person can only scream so long. If she doesn't wise up in a few days, strip her room of everything but the minimum essentials when she is gone for the day. When she returns tell her she will start getting things back when she can stay calm for three days in a row. It will be a while, but as she masters this, you can increase the time a day at a time up to a week.
You and your wife must also resolve not to yell or lose your temper. You decide what is right or wrong, and then stick with it.
It won't be easy. You've lost a lot of ground and you don't have many years to make it up. Frankly, most people have too much ingrained habits and are not inclined to change their own behavior. Instead they look for a magic pill that will change years of bad raising into sudden righteousness. It just doesn't work.
"He who begets a scoffer does so to his sorrow, and the father of a fool has no joy" (Proverbs 17:21).