I need a break...

I am having a rough time of things lately.  Matt has been working a lot and thus isn't home to really help with the kids/house/dog, etc.  Jack is out of school for the summer so we are getting used to a new schedule and a lot of time together.  Anna is developing rapidly and while she amazes me everyday, she also knows just how to frustrate me.  I love my kids, I really do, but I feel like so much of my day is yelling and saying no.  I need a break, both figuratively and literally.  I don't really know how to do that though.  We can barely make it every month so how can we afford for me to go off and have a "break"?   I don't even know how to give myself a break figuratively.  I barely have time to pee by myself, how can I take the time to figure out how to relax?  I am hard on myself, I know I am, I have always been that way.  I have a hard time forgiving myself too. I just feel like I am not being the best mother I can.  I probably yell too much of the time.  I worry about breaking my kids spirit.  They both are so amazing, each in their own way.  How do I remember that when they are driving me crazy, not listening, getting into everything, and just being destructive??