Oh Papillon . I don't think any of us is perfect and the times I have lost it and yelled at my DD just seeing her little face crumple is enough to make me feel like I'm a monster.

You're right, she is picking up on your emotional tension, and you are acting out of yours. But the emotional issues are not going away right now, so you probably just need a few strategies to keep your cool and hers. I am certainly not an expert, but after a terrible time with my DP last year in our relationship, I found a few things that worked to keep things cooler, at least when it was just DD and I.

Tantrums bigger than we have ever dealt with, Toileting issue's, general nastiness and being rude that is just not like her. She is normally a very polite, gorgeous caring little girl.
I think you'll find when you settle down and deal with her differently then this will ease. Don't worry about the toileting and be gentle with her when she's 'nasty'. It's hard to process emotions when you aren't really sure how to. If she was learning to walk, you wouldn't get cross with her for stumbling. It's the same with emotions. I find when I remember this when DD is having a meltdown it's easier for me to keep my cool.

You know smacking and yelling is not helping, so try not to do that. I'm not saying you won't get frustrated or annoyed, but walk away rather than smack. She might not like that, but YOU need a time out if that's how you're feeling.

The second i say no to something she requests or wants, she screams so loud you CAN hear it from the road,
I don't tell my DD 'No'. I don't give in, but I find an alternative that is truthful. So, a request for something she can't have gets a "You can have that after such-and-such/tomorrow/You had that only a little while ago, how about something else now?" Try to think of a different way to phrase your answer that doesn't start with 'No'. You'll be surprised what a difference it can make. You say she used to be good at reasoning with, so start a conversation with her and ask her why she wants what she wants and then explain your reasons for not giving it to her. I don't put my DD off with a 'maybe later' that never happens either.

she throws herself around, she starts being mean to either me or DF or her brother... by saying "your not my friend anymore" "i don't like you" or mimicking how we have been disciplining her by threatening her, or taking somthing away or whatever....
"I'm sorry you're feeling frustrated and hurt, would you like a cuddle?" Works wonders here.

After i have smacked, or yelled at her, i feel just horrid to the core. I feel this immense sense of guilt its just not funny... i am even stretched so thin i think, today when she was having one of her tanties, she said "go away" to me and somthing else that wasnt nice, and i replied "ok i will, i will go away and i wont come back"
We all slip up and behave in ways we'd rather not have. When I've yelled at my DD or done something else that I think wasn't right I get down to her level and apologise. I give her a hug and say "You didn't like it when Mummy did [whatever], did you? It wasn't very nice of Mummy and I'm so sorry, will you forgive me?" We cuddle and go and do something fun then to get over it. I don't hang on to the guilt either by doing this, because then I feel that she understands we all make mistakes in the way we treat each other, but it's resolving them that's important.

Hope that helps and you feel better soon. Take care of yourself, hun. That's going to be key to getting back on track.