HELP.

I am feeling so deflated and the thoughts of "where did I go wrong" are constantly going around my head.

We are having some issues with DD who is nearly 3 1/2yrs old.
We have tried a multitude of things,
time out
praise
time in with us
going to her room
smacking
rewards charts
talking about behaviours we expect prior to them happening

and nothing seems to work. Today we went to look at new child care centres and she refused to get off the swing and basically just says NO.
She refused to say good bye to the lady at the centre or say hello (SO embarrassing )
She will yell while I am on the phone in the car or at home because she wants attention.

I know alot of this is age based developmental pushing the buttons but I just don't know where to go next. Discussing things with her and explaining why it is nice to say sorry/hello/goodybe doesn't seem to have any effect.

Please send me some reassurance this will pass or some ideas of how to handle this challenging behaviour because I am tearing my hair out and hating the way I am yelling and parenting right now.

ETA: She is generally the most gorgeous little girl, amazing imagination and plays really well with James and by herself, but it's just when she digs her heels in on stuff like this that I don't know what to do.