Sibling Fighting

Ok parents, I am not kidding here, I am going to have a break down. My 6 year old son and 4 year old daughter will NOT STOP fighting, they fight about everything, and everywhere we go! How do I make it stop?!!! I also have a teething 21 month old in the mix. We are out at the store and the two fight over things they see at the store and touch, they try to take the things from each other and push each other out of each other's way and then there is screaming and whining and I am the frickin helpless embarassed idiot mother with the screaming fighting kids and I just want to run and hide in a corner and literally pull my hair out and scratch my nails so hard on something that they rip out so the pain of that helps me forget the embarassement and stress my children put me through!