Not getting a break is tough. On the really difficult days, just keep focusing on the blessings you have. Don't keep mulling over and over in your head about how difficult your day is. It will make it worse. When you absolutely need a sanity break, let your daughter spend time in her room playing and put your 1 year old in a crib or playpen with some toys. Kids need to learn to entertain themselves instead of running to mommy and saying, "I'm bored!" When they tell you that, just tell them to go find something to do. Do not become their answer to boredom.
You also need away time. Plan a night each week when your husband watches your kids and you go out with a friend. If money is tight, don't go out to eat, just go for a walk or go window shopping. It is nice to have a set time that you can look forward to instead of wondering how you are ever going to get through this stage.
Since you know ahead of time that you will hit those crazy moments, plan ahead how you will respond to it. Having healthy responses in mind will keep you from reacting in ways that you don't want your children to mimic(screaming, etc.). For example, you can turn on some fun music and dance. It will get the pent up energy out in a fun way. The kids will probably join you!
Hang in there! Keep reminding yourself that it could be worse. And when they are finally asleep at the end of the day, literally pat yourself on the back and tell yourself that you are the best momma they got to have today!