I think the biggest thing you can do to help is try not to lose your temper, when you do you've lost control you put the kids in charge.

If you feel your getting to the point of shouting or crying walk away, there not doing anything you ask of them anyway so them not doing it for 10 minutes while you take a deep breath isn't going to make any difference at all. Go back when your calm and say again what you want from them, walk away a hundred times if you have too. Once you can stay calm you will be back in charge.

They are winding you up and winning atm, they know how to push your buttons so show them it won't work anymore.

If you do lose your temper replay the moment in your mind later when the kids are in bed, think about what you could have done differently...practise for another time.

Silence is always a lot more effective than shouting at them and a lot less stressful for you. Make some time for you too, time to relax and look after yourself so your ready to face another day.

They shouldn't have to share. You don't share either. They grow up in a society of materialistic people who don't share their cars, computers, food, money... other than with select relatives and friends and in a very limited way. We never share what we don't want to share: Why should they?

Honor their ability and provide privacy and ownership. Buy two of each toy, fewer toys, etc, and model more sharing.