Struggling with ways to manage my daughtets behaviour

My daughter was recently diagnoaed with ASD. She is 3 years 9 months. We are waiting for her last appointment to find our where on the spectrum she is. Ive been working with her nursery teachers but we are all at a loss with her some days. She has periods where she is brilliant and listens and tantrums are few and far between. Then we get weeks of constant screaming and hitting. She tells people she doesnt like them quite often and is really hard to control in public which has become a danger around cars etc. Ive tried many techniques to try and help her but nothing seems to get through to her and me and her dad are at our wits end. How can i teach her right from wrong when she wont listen to anything said to her. She shows a real lack of empathy unless its for herself. She instantly cries when told no which can lead to a major meltdown. Every day is a battle alately. Any advice from someone who has similar issues? We love our unique little girl and just want to do the best for her