This will be long and ramble-y. Please feel free to skip. 
For those of you who had less than stellar childhoods, how to break he cycle?  
****PLEASE DONT QUOTE*** 
My parents were drug addicts and spanked me. My dad used his anger to manipulate me to the point that I was afraid of upsetting him EVER. It was a super unhealthy dynamic. My mom was a yeller. Any little thing would set her off and there was no stopping her. 
Don't get me wrong, I LOVED both of my parents dearly and wish I could turn to them. But I don't want to make the same mistakes, and as I'm being slapped in the face repeatedly by my 18 month old DD, I feel the urge to scream at her. She's a baby. It makes me wonder wtf is wrong with me? How can I change that urge? I feel like a world class asshole for even feeling this way. 
Tl;dr, I'm messed up and need advice from others who can commiserate.                 
                             
        
Re: Breaking the cycle (heavy childhood content)
2. Reading and trying to understand my past
3. Mommy time outs
4. Xanax (not even kidding)
5. The ladies of TB
Good luck, and as PP have said, recognizing it is awesome. Sending you lots of hugs. Kids are jerks and at least you only have the "urge" to scream. I scream. I don't like it, but I'm a work in progress.
You're such a sweetheart, and I know you're an amazing mom.
Also, allowing yourself the opportunity to walkaway when needed is a great piece of advice as well. I'm sorry you're struggling. But don't ever feel like a bad mother. The fact that you are aware of this means you're an awesome mom. Hang in there.
My parents were pretty absent but my mom dated abusive assholes. I have a lot of anger. My initial reaction is anger sometimes. It fucking sucks. I hate being so upset and angry. Therapy for three years has helped. And then just focusing on how I'm feeling. When she does something that upsets me I ask myself why I'm upset. And then I ask myself if it affects me. Is it really going to ruin my life if she dumped all of her crayons out over her bedroom? Is it going to matter tomorrow if she asks me twelve times if she can have candy after I told her no and to not ask again? Even though I'm frustrated, usually it's about trivial things and I have to recognize that and even though my feelings are really strong, I control my reaction. Even though I'm overwhelmed and want to yell, I try (and sometimes fail) to walk away/ask for quiet time/have her do an activity without me. If I let my emotions get the best of me I apologize.
It is far from easy and it takes awhile to change how your brain reacts, but you are a wonderful mother for caring enough to want to change and stop the cycle. You are a wonderful mother whenever you try. Take it one day at a time. One moment at a time sometimes. And give yourself credit for doing something difficult.