Need help... failing as a mother :(

I have 4 beautiful children aged 8, 7, 2 and a half and 9 weeks old. I used to be a great mother, everything was perfect. When I just had my two older boys life was great. they are great kids were always so well behaved, I was always so chilled out back then and life was easy.. Then my daughter came along. everything was still great up until my daughter was about 1 and a half. I don't know what it is... maybe it's because she is a girl, I don't know but she is so full on and so naughty. she keeps me on my toes everyday, doesn't listen to me at all, screams, hits, spits, she is a little terror. Now my baby has come along and he's a great baby but of course I still have my hands full. I've turned into such a shitty mother and I don't know when it all changed.. I speak/yell at my oldest boy in such a horrible way. at the end of every day after they have gone to bed I sit there in tears telling myself tomorrow's going to be a better day, your not going to scream at him or talk to him nasty. tomorrow will be better. then tomorrow comes and I start off good in the morning. and then he just doesn't listen, talks back to everything I say, I have to tell him to do the same things everyday at least 20 times it drives me insaine to the point where I scream at him.. sometimes in his face :(. I feel bad about it immediately but he just keeps going... gets the biggest attitude with me starts slamming doors etc all while STILL not getting himself ready for school. I'm trying to make lunches, the kids breakfast, tend to my baby, deal with my toddler, get the kids ready for school, get myself and my, daughter dressed, clean up and so on... I just get so stressed out and it all gets vented towards my son. alot of the time it's my daughter screaming, playing up, being naughty that gets me angry and I end up directing my anger at my son.. I feel like such a shit mother and I want to change it. but how do I keep my cool!? on the rare occasion when it's just me and him it's amazing I love those moments we get along so great but sadly we don't get that opportunity very often anymore. I used to be so chilled out but that is so not me anymore. I know I'm going to get alot of hate and yeah I deserve it but I'm trying to reach out and get some advice. some stress coping methods? ways to calm down when I do find myself wound up? My husband is gone before the kids wake up and doesn't get home till after they have gone to bed so during the week I do it all on my own.