Dear J.,
I wish I could reach right throught the computer and give you a hug and tell you it is alright every Mom feels like this sometimes!
I read the advice you've been given and it is all good advice that worked with my now 5 year old daughter. When she was 2 I was pregnant with her little brother and I was losing my patients all the time, yelling, shouting, slamming things; in fact acting like a 2 year old myself. My mom reminded me that my daughter was just a little girl and to choose my battles. Whispering worked for me, as well as "toy time-out", pick-up and carry if you won't hold my hand (they actually don't want to be carried sometimes at this age), and immediate consequences for their choices; "are you making a good choice or a bad choice?" And always saying, "I love you, no matter how you behave or what you say to me". I read every Dr. Brazelton book I could find and it gave me wonderful tools and clearer understanding. But I am more worried about you!
I worry that you feel trapped! Maybe I am projecting but sometimes I felt/feel like my life is only kitchen duty, laundry duty, driving duty, referee, and disciplinarian. I had isolated myself in my home all day while my partner went to work. I needed a break, not much just an hour here or there. I joined a Mom's club and met other moms and could vent while my children tired themselves out for nap. I am now taking a night class for 1 hour once a week, something fun - ballroom dancing.
Please, please, please schedule one hour a week, just one, of "me time" for yourself (you can add more time when you see it can be done). You think you can't but you can; swap with another parent if you can't afford a sitter. Call on a trusted friend or parent or sibling. Go for a walk, go to the library, it doesn't have to cost any money. It will do you and your daughter a world of good. She needs time away from you just as much as you need time apart, remember "absence makes the heart grow fonder".
I believe in you, J.! You are strong and loving and you can develop the tools to enjoy your life and your daughter. Just make "me time", just do it!