Good at giving advice, bad not at taking my own...

I am such a bad mom. I'm reluctant to follow through, try to negotiate, constantly give warning after warning, etc. everything I know is not good parenting. My biggest problem is that I am lazy, tired, frustrated, and let my emotions be a part. My kids know they are pushing my buttons and I can't seem to bluff it. They are good kids, and generally don't misbehave, but when they do it's a large struggle. I do always come out on top, but after the screaming crying long temper tantrum and the rest of the week being one long authority struggle. I feel like I have to yell, which I know doesn't work and is counterproductive. The other problem is I just don't have an authoritative personality or demeanor. I'm just "not scary". I'm short, and apparently cute and loving and made to follow, not lead. How the heck am I supposed to change that?! I've tried different inflections of my voice. months at a time of different reprimands, but I just can't seem to get it down. What am I going to do when they really start thinking on their own?
But I certainly know the theories behind good parenting, why can't I follow them into my home?

How old are your kids?

2 and 4.

Sounds like you need those tv english nannies on nanny-911 and stuff. The make charts and give stickers and put the kids on the naughty step (in time out).

I loved watching those shows when I was living with my mom. I was in my 50's and my mom was in her 70's. I would tell her not to yell at me cuz it hurt my self-esteem, according to the show. Plus those kids made me look good, I wasnt nearly as bratty as them!! Then we would threaten each other with the naughty step. It was like the Grey Gardens of the Midwest.

Short, cute can be scary . I think it depends on what your kids are doing and how you want to discipline them. It sounds bad but time out for my son until he understood it was a play pen with the time out chair in it. It gave him a safe space while we started cleaning up but with us time out didn't start until he stopped talking back. We had a timer that beep. Time out when he was little was to keep him safe.

Now that he's older he has to write down why he choose the action that got him in trouble and solutions for solving the issue without getting in trouble.

from watching my nieces and nephews growing up and watching lot of those nanny shows, i think the key to your children respecting your word is follow through, if you threaten them and nothing comes of it, they soon realise the is no weight to the words.
you dont need to shout (but children can make us so frustrated) you need to make them understand that if you say they will be punished, then they will be punished.
I know that trying to set a new standard in your home is going to be a very turbulent time and the kids will try push the boundaries as far as they can. But u must stay strong, and don't forget to reward good behavior with time and affection.
Just my thoughts on what i have noticed/done while babysitting, i am certainly no expert but maybe give it a try?