Anyone here have a child that doesn't stop?

I have a darling 4 year old. I’m a single Mum.
My four year old seriously does not stop. It’s getting quite hard for me to manage, I suffer from anxiety as it is, but she seriously never slows down. She is always on the go, she has no awareness of stranger danger – even though we’ve discussed this. If we go out to a cafe she says hi to everyone, will tell everyone to watch her, dances, sings and does everything she can to get attention. At home, if something doesn’t go her way she will throw a full on tantrum and scream only next minute to be fine and she’ll laugh at me for trying to help her. If I explain to her that sometimes her yelling hurts me she’ll just laugh and say, ‘I don’t care.’
She’s an incredibly bright little girl but I’m at my wits end to understand how and why?
I’m very compassionate yet she seems to lack compassion – her school teachers have passed that by me as well. I’ll ask her to be quiet when we are out and to respect other people are enjoying their time but she doesn’t listen at all, in fact, if I ask her to be quiet she’ll yell even louder and she’s started repeating herself to a point of me thinking she’s slightly ADHD. She’ll point at something and say, ‘Car, car, car,car, car,car ..’ over and over again and I’ll respond, ‘I see that darling,’ – then I have to literally say, ’ OK STOP!’ – I’m not an impatient person but I’m exhausted, not a lot of support as no family and I’m feeling a bit worn down. Her pediatrician at 6 months told me she’d be a high needs child, whatever that means – and how would she know at 6 months, but she’s definitely high maintenance and this poor Mummy is exhausted and scared on how to get through another day with patience and understanding… any thoughts?