I don't have any good advice for you b/c what you described was me. Actually for me things have been better b/c I am now on medication but there were more underlying issues there.
First I disagree with you that you are a bad mom!! You are an awesome mom b/c you are recognizing something that you don't like and you are making an effort to correct it. I know exactly what you mean about the parenting books, I feel the exact same way. I can't read parenting books until I get myself in line first.
For me I try to find out what my triggers are and avoid them or anticipate them. I get crabby and super short on patience when the kids fight me on their naps for example. I am much more likely to fly off the handle if they are fighting me because I really want my time alone! But I am able to recognize that as one of my triggers and try to do things to prevent that from happening. If it does happen I am able to remind myself to take deep breaths and recognize that is is just one of my triggers and cool down.
If I feel myself in a stressful situation where I am starting to get angry, I try to figure out a way to cool off the situation. If my 2 year old is acting out of control and I know I am going to loose it, I will put him in his bedroom for some quiet time by himself so that I can cool down and regain composure. Or I will put myself in a seperate room long enough to re-focus.
It is a hard daily struggle for me, something that has been ongoing. I keep plugging away at it, and try to laugh when I want to yell. I try to remind myself that I don't like it when people are angry with me, and I really don't like being yelled at. There really is no reason to yell at your kids, all you do is teach them to yell back.
I guess the best advice I would have is to just walk away if you feel that you are going to loose it. If I know that I am going to flip out on my dd for something or if she is just pushing my buttons over and over again, I know the best thing for us is seperation. Also make sure you are getting enough sleep,and good food. Also for me atleast, even an hour alone w/o children in the evening can go a long way. Or a trip alone to the grocery store.
Try not to feel guilty, you are doing the best that you can and you are making an effort to improve your parenting skills because you love your kids. That is So important, and your dd will recognize that you are trying to do better.