Idk what to do

So my 3 year old does this thing when he demands something. We gently correct this by saying how do we ask nicely? He then goes please. Then I say can I or may I ____ please? And for the most part he repeats after me. It's a slow process. But then he does this thing when I tell him to stop doing something or not to do it and he keeps doing it. Then he wants something and I say he can't because he isn't listening. The apologies for not listening and not stopping. But continues to do the thing I told him to stop doing. Then he threws a huge fit when I tell him no he can have such thing. And of course he says but I said sorry. I tell him thank you for saying sorry but it doesn't mean you will get your way. I am trying to teach him. Maybe I'm being too harsh on him but the not listening is stressing me out.