I cannot really give advise as I am pretty much going thru the same thing with Liam at the moment (thank goodness Brandon is old enough to understand that right now Liam needs extra attention and gives me the space I need to deal with it).
But, please do not feel that you have failed as a mother, children need confident parents in order to obey - they smell the fear and react to that

Remember YOU are the BEST mother you can possibly be for your children.
I read copious amounts of books, trying to find solutions that work for me, but you need time and the concentration to absorb the lessons books offer. And I sometimes feel that books do not offer the comfort & sympathy that a therapist offers
So I found that chatting to a therapist has helped me more than all the books I read. You go back every week and discuss the methods used and then "tweek" them to be more effective. You can ask questions (which you can't when reading).
He also spent a few sessions with each boy and offered very different advise for each child, according to personalities for eg Time out is effective for Brandon, but does absolutely nothing to control Liam. Spanking controls Liam, yet scares the living daylights out of Brandon.
So Brandon gets time out, Liam gets a hiding. And I know this seems unfair towards the children, but at the moment it works for us.
As they grow older, so we will adjust the discipline methods.
Another thing he has taught me, is to put myself in time out - when I feel my temper rising - I lock myself in my bathroom, close my eyes, block out the screaming and breath deeply, regrouping myself. When you are in control of your emotions, you are better able to deal with theirs
And once again - YOU ARE NOT A BAD MOTHER, YOU HAVE DONE NOTHING WRONG !! Let this become your mantra and every time you feel down, repeat as many times as you need.