I read your question from when your daughter was 5 1/2 years old about the way she acts with you - fighting, stomping, etc. You said she was an angel with everyone else.
I have a feeling that the issue is behavioral and attention seeking. I'm sorry, but I think it's just too easy to define this as an anxiety disorder. If you classify this as a disorder, then she gets a lot of attention for unwanted behavior. I think you are setting your child up for some really bad traits as an adult by giving her an "out" based on what you "think". Instead, you need some parental guidance on how to "train" her to stop acting this way.
If you don't think that children this age can manipulate you, you are wrong. They don't even know what they want - they just know that if they push the right buttons, they can get the adults around them to worry their heads off, change their minds, try this and that and run circles around them. Truly, children need limits, L.. They need the adults around them to say "Yelling at your friends during play date time is not nice. If you treat your friends badly, no one will want to play with you. You have to go to your room and stay there without coming out. Go right now." And if she has a meltdown, you IGNORE IT and make her go to her room, period. You send the friends home.
You don't show her that you feel sorry for her during her meltdowns. You immediately send her to her room, make her sit by herself in the car while you wait out her tantrum standing outside, reading your phone or book and ignoring her. At some point, she will want you to pay attention to her and she'll stop the behavior in order for you to want to be with her.
It's no fun for a kid to have a meltdown when no one is paying attention. You don't stand outside her door so that she knows you're listening. If she comes out of her room before her time out is over, you march her right back in there over and over until she stays. She never gets to come back out unless her tantrum is over and what you have told her to do is done. (That includes the amount of time you've told her she has to spend in there, on top of her stopping the tantrum.)
As far as activities outside of the home are concerned, if you are going to quit an activity that she has meltdowns over, I would recommend quitting what she always says she doesn't want to go to. Sounds like you're giving in to her? Well, here's the rub... the activities she likes is what you use as her "currency". You tell her when she starts up with a meltdown that you are going to give her 3 chances and if she doesn't stop the behavior, you will not take her to her favorite class that day. AND MEAN IT. Tell her "That's one. Calm yourself down or you won't go to soccer." "That's twice. Calm yourself down or you won't go to soccer." Okay, that's three times. No soccer today. Go to your room now and don't come out until you've learned your lesson about calming yourself down."
It doesn't matter that you want her to go to soccer. It doesn't matter that SHE wants to go to soccer. What matters is that you have to demand from her better behavior and she has to learn that you mean it. If you give in to her and let her continue to manipulate you by behaving badly, she will just continue to and you will all be miserable. Kids who do this over and over lose control over themselves because they are not made to learn that they will lose their privileges without fail - mom will turn around and drive home rather than going to the park - dad will turn around and drive home rather than going to the movies, etc and etc. And every single time the kid whose behavior messes up the rest of the family's plans will have to go to their room for a long time as soon as they walk in the door.
If you don't at least try to do this for a good chunk of time, you will be doing your daughter a big disservice. She will up the ante and act worse while you put your foot down, trying to get you to abandon this course of action. Right now she holds all the cards - she's getting you to do what she wants -pay attention to her regardless of her behavior. You need to take the reins now and make her understand that you say what you mean and you mean what you say. Failure to do what she is supposed to do just because she says she doesn't want to makes her lose her privileges to do what she wants to do. She will eventually learn that SHE is the one who has to say yes to doing things.
I get the feeling that you are wondering if you should stop sending her to school and homeschool her. I hope that's not what you're starting to think. If you don't try to manage this now, you're going to be having these meltdowns full time.