Horribly behaved child! HELP!!!

I just want to know, is there an end to this terrible 3's?
I feel like im going mentally insane!! I can't control my 4 yr old anymore, she rude and so nasty, she sooks, crys, carries on and never ever listens!! Shes embarrassing and mentally draining, everything is about her and what she wants and If she doesn't get it she lets everyone around her know how annoyed she is, like yesterday at the in law's one of my MIL's friends came over as her mother just passed 2 days ago and we were all there to support her, well she sat in "her" seat didn't she and it was the end of the world even though she hadn't sat in or gone near the chair the whole time we were there!! Soooo she carried on for a good 25 min before i snapped, She kept telling her to move, crying, trying to push her off the seat say that she "wants its", she even started screaming in her face so loud it turned her face 5 different shades of red. So i pulled her aside, got down on her level and said calmly "we dont get anything by saying "i want" we need to use our manners, she is a guest at this house we need to be nice and share then when she leaves you can have the seat back" but she got worse!! she threw her self on the ground started kicking and scream, it all ended in me dragging her up the stairs to put her in "her" room (I've tried everything from smacking to naughty corner but seperating her from the situation is the only thing that makes her calm down)... now this is just ONE example but she dose this to everything and has been for months and months, sometimes 4-5 time's a day and personally i just want to break down and cry!! A yr ago i had a perfect little girl, she did everything i said, wouldn't argue, used her manners and was the most well behaved little girl anyone has ever seen, I'm talking about a little girl that got a box of chocolates for christmas and shared it with every person at the christmas party with out me asking her to!! I used to get compliments daily about how beautifully behaved she was but now.... WELL she's the devil child from hell sucking the life out of me with every passing day, i get horrible looks where ever i go, people shaking their heads at me and even rude comments on how bad my parenting is. Now im done! I dread getting out of bed, i dont want to be with my partner any more because o feel bad that he has to deal with her (he isn't her father)... i find myself just wanting her go away and sometimes when she is really bad i even find myself hating her and regret becoming a parent! Not because of her, i love her, i just cant handle her moods any longer and I feel horrible for feeling this way but im literally DONE being a parent and its not fair on my other children either (i have 3, her, her brother also 4 and her other brother whos 8 weeks old). Her 4 yr old brother dosen't behave anything like her and my 8 week old is a deam baby already sleeping through and never crys and always smiles. What have i done wrong??