How do you cope when you get angry with your kids?

Hi, i'm a single mother to a 5yr old boy and a 5mth old girl. I love my kids more than anything in the world and they are the most important things in my life but i seem to get to a point recently where i am so stressed that i end up shouting at them. I am two weeks away from my final exam for my degree (finally i can get a proper job!), have coursework to do as well as the housework and looking after the kids and have just come down with a cold. With all of this combined i am shattered, stressed and have noone to help me when i need a break. It got to the point earlier where my son was pestering me about some toy, my daughter was whining because she refused to have a proper nap in the day and i lost it and shouted at both of them. I am horrified with myself but i just got to a point where i couldn't take anymore. I feel terrible and have spoilt my son to compensate and cuddled my daughter non-stop until bedtime. I am sure i am not the only person who finds it hard sometimes and i am just wondering if anyone has any tips on what to do when you get to this point as i dont EVER want to shout at my kids just because im tired and stressed again!