Poor you. I know that behaviour really well. We are going through it at the moment.
Does she go to nursery/pre-school? My DD started the "you're not my best friend" thing after hearing the other girls say it. In fact, they are all going through a phase of it and everyday it's a different person they are not friends with anymore. I just ignore it. If she says it to me, I just say - that's a shame because I love you very much and move on.
With misbehaving. We have a 3 chances rule. She is told not to do something eg - don't jump on the sofa. 2nd time she does it, she is warned that if she does it again she will have to go to her room. Third time it happens, there's no fuss she is just told that she was asked not to jump on the sofa and she continued to do it, so she has to go to her room (haven't quite got passed her slamming the door yet
) and that she can come out when she is ready to apologise and behave. She has never stayed in her room for longer than a few minutes, before she comes out and apologises.
If we are out an about doing something, same principle applies, but the consequence of bad behaviour is generally we stop whatever we are doing and leave.
This didn't take long for her to understand that there are consequences - only a couple of times, and generally the 2nd time of telling, along with the consequence is sufficient for her to do as she is asked.
As for tantrums, we're quite lucky and DD is generally distracted. If not, and reasoning won't work, then I just let her get on with it and carry on with whatever I'm doing and then as soon as she starts to calm down, I give her a big hug.
You're not alone. I know a lot of parents with DDs this age and they are all experiencing the same thing. I believe, as MN says "this too will pass". And hopefully, you'll still have your sanity 