Maybe just say that she is acting age appropriate & that all kids her age do that stuff.

She doesn't know right from wrong yet. She needs to be taught. If she gets a smack, she will think its right as you guys are who she is learning from.
That means if someone at daycare or somewhere does something she doesn't like, she might respond with hitting/smacking herself as thats what she knows. If you keep up with the talking to her, as she gets older she will see that as the right thing to do.

If she see's a kitchen cupboard she doesn't look at it & think 'Mum won't like it, but it looks like fun.' She just thinks it looks like fun. She could do it 5 times, but can't really comprehend at her age that every time she does it she will get in trouble.

She won't understand that til she's about 3. My DD is just 3 & still doesn't quite understand. My 6 year old on the other hand...But thats another story!

All she sees is what is in front of her. She can't think ahead to when you see her doing it, & she can't really remember what happened last time she did it.

I hope you understand what I'm saying. I'm not even sure I do, but if you do & if you both keep that in mind it will make it easier.
You might still fly off the handle, but before you finish your rant you will remember that she has no idea what on earth you are yelling aboout!
Even if you try explaining why its wrong, she still won't quite get it. You just have to persevere. If she pulls things out of the cupboard get her to help you put them back. If she does it a few more times, maybe get her to put it away herself.

You will get there. All of you. It will get easier. After a while.