I never felt like a natural parent either tbh. I have 3 DCs but still feel like an imposter and that someone will realise I should never have been left in charge! However, over the years, I've found what works and what doesn't.
I don’t feel like I know what I should be doing with DD at this age, how to teach her appropriate self care etc (we seem to have stalled with progress with getting dressed etc, her diet is not great...)
These things come down to boundaries - do you struggle saying no to her if she won't get dressed or won't eat certain things etc? I found with my first DC that it was easier for him to get his own way as it was only him to worry about, and if he didn't get dressed etc there weren't really any consequences for either of us! Although when I went back to work he had to start being more cooperative with getting dressed etc!
By the time the next kids came along, they had to fit in and I didn't have time for tantrums or to molly coddle them over food etc, they ate what they were given or went hungry, they soon learned. If they didn't get dressed, we had to leave anyway to get DS1 to school on time, so they got taken to nursery in their PJs - only happened once and that was enough!
Books wise, I was given Dr Spock by my parents as they'd had it when I was a kid, but that was mainly about health more than happiness/behaviour.
I also read an old 1970s book called How to Parent when I was younger, (way before even thinking of having my own kids - not quite sure why!), which explained a lot about the development of little minds. Some child psychology is a definite help to understanding why they act the way they do!
The concept of First Adolescence and Second Adolescence was an eye opener for me. The idea that the transition from babyhood to childhood was as much of an upheaval as that from childhood to adulthood. While we all understand the mad hormonal rushes and battles of wills to expect from teenagers, we don't necessarily realise toddlers are facing the same struggles! I think if you can reframe any challenging behaviour as coming from a place of trying to assert power, you can battle it appropriately!
"Put your shoes on"
"NO!!"
Becomes "Lets get our shoes on and we can get to the park, what do you want to do when we get there" and then the emphasis is taken off the demand to do something and becomes about giving her choices, so she feels she has the power. (But actually she doesn't!)
A bit like saying "do you want to wear the blue top or the purple one?" , you acknowledge that she has a choice, but you limit that choice so that she can only make a good one.
Its not easy and even those who look like they know what they're doing probably still doubt themselves. We're all just doing our best. 