It sounds like you are really engaged in a power struggle. After teaching for ten years, I've concluded that some kids are only intrinsically motivated. These kids also like to be in control of their own destinies (don't you like to be in control of yourself?) Recent research has confirmed that for many kids rewards AND punishments are not really the motivators many parents believe them to be.
For kids like this the things you are doing make things worse because all your battles become about control instead of learning, correcting and genuine discipline that comes from within the child in the end. I am not saying just to give in and let her have her way, but I am saying that perhaps a punishments/rewards system could be abandoned for life lessons. If she is throwing a fit, I would say something like, "Wow, you are so frustrated that I can't even make any sense of what it is you are saying - when you are ready to speak like a big girl, I'm ready to listen."
Take on the role of listener and guide with your daughter. If she is speaking calmly, listen calmly. Ask clarifying questions and paraphrase her. by paraphrasing you will let her know you understand her side. Then, you can tell her your thoughts. Obviously, as parent there will be times when you will need to just make decisions for her, but there may be times when you can give a little too.
For example, maybe instead of buying her toys, give her an allowance (not as a reward, just an allowance). Then you don't buy her toys except at important holidays and her Birthday. If she wants a toy she will makes the choice of what she buys and what she doesn't when you are out. If she can't afford something, she can't afford it and it is a true-to life lesson. In the meantime, she will make mistakes and buy some things she regrets buying - empathize, but don't try to help her "fix it" or return the toy or give her money to buy the other thing she'd rather have now. She needs to learn the lesson and by not saving her she is able to learn the lesson as well as learn that while you are there as a support and a guide you are not controlling her either.
For most things, natural consequences like this are much more likely to help better. You might have asked her to help set the table for dinner at some point. As an example of how this style might look if she were to refuse helping is to handle it as follows: If she isn't willing to help get dinner ready, start by asking her why? After you've shown her you are willing to listen, state your needs. Maybe she is saying no because she is going potty, or maybe she'd like a couple more minutes so she can finish the picture she is coloring - no big deal, but she needs to set the table before (set time limit). If she still isn't willing to help, then eventually, she shouldn't get any of the stuff you cooked. I might start with a response like reading "The Little Red Hen" first and escalate from there. No one wants to continue to serve someone that doesn't do her fair share too - including Moms and Dads. Eventually, after you are sure she understands this connection, you can give her a piece of bread and let her figure out how to make a sandwich or toast, but you've put effort into preparing a meal and she needs to share in the preparation some way, if she won't set the table, she doesn't get to participate in the eating.
Same thing with cleaning. If she doesn't put away her toys and you have to clean them up. The toys are no longer available - not because you have taken them, but because in real life, leaving stuff out and around results in those things being broken or lost and the room unusable. Since other family members have to use the room you are just putting the toys away where they are out of the way of the other family members (then tell her how she can earn the toys back. The method should be related somehow to demonstrating her willingness and ability to take better care of her things).
The thing that is super important with this method is that you ALWAYS REMAIN VIRTUALLY EMOTION FREE. Except when expressing empathy. Consequences arise because they are the natural order of things. Consequences (good AND bad) not arise because of your anger, frustration or even because of your happiness in her choices or pride in her successes.
If she makes a poor choice, feel badly with her about it, but don't jump in and rescue her from her own poor choice. Be there for her to talk about ideas that may have worked better for her that she can CHOOSE to use next time. It is a difficult thing to switch to and requires a lot of work but gets MUCH better results in the end. Check out schoolathomeeffectively.com for more ideas about how this method actually works out in a real home.
In regard to the question about seeing some one. Go ahead. Why not? A therapist can help the both of you manage the stress in your relationship and make sure there isn't anything deeper at work here.