Oh your post could have been written by me. I'm so sorry you are having such a tough time.
My ds is very similar, ie strong willed and at the moment increasingly defiant and I get fed up of people telling me to put my foot down etc and even worse, telling him off etc in front of me rather than letting me deal with him myself.
"It feels like it's all getting on top of me today and I don't know what to do for the best with him. I don't want to put all the behaviours down to his autism but how do I know what are due to the autism and what are just him being an awkward and defiant little monkey? How do I get it right?"
I said the above paragraph almost word for word to my Mum and my dh at separate times today. I really struggle sometimes to know which behaviour is just ds being naughty and which is him struggling to cope and while I don't want ot punish him for things he can't help, I also don't want him to get away with things that he is able to learn and control.
I suppose its just trial and error and then building on what works, but my God its tough going through it on a day to day basis and some days it feels like its all just too hard.
I totally agree with Marne's advice to make sure you get some time for yourself (I am hopeless at doing this though so I do understand how hard it can be).
My escapism involves Mumsnet and a large bar of chocolate .
Incidentally, my ds only gets invited to 'whole class parties' as well and his 'special interest' is his Nintendo ds which he can bore for England on talk about for hours. One of his major goals in life is to find people to play two player games with him on it and he only really talks to the other children in his class that also own a ds. His favourite game du jour is Pokemon Heart Gold, followed closely by anything that has Zelda in the title.
Very un MNetty hugs to you.