Hi! I can certainly empathize, as my 3 1/2 year old daughter is strong-willed as well. However, as we seem to have just cleared several hurdles in her behavior recently, I'll offer what has worked for us so far, understanding that parents and kids are all different: I try to figure out what she really wants/needs, then teach her an APPROPRIATE way to get it, or if it's something she can't have, give her some kind of coping strategy for dealing with her frustration. For instance, she was terrible about hitting her new baby brother whenever he cried, and sometimes me. Of course, his crying stressed her out and took all my attention; I also think she took it personally. My response was to immediately send her to her room (I don't care if she plays in there--it's a time out for me to calm down, to give her brother a break, and to give her a break from his crying--if you think about it, it's the same thing that would happen to an adult who can't handle behaving appropriately around people for whatever reason). After a decent interval of time (depending on everyone's mood, quite frankly), I'd go ask her if she's ready to be gentle to her brother and apologize to him (and/or me). After that's all done, and this is the important part, I try to find something she can do WITH her brother under my supervision (pattycake, tickle toes, sing to him, etc.) so that she has a replacement behavior--a way to interact with him that's okay. I'll be honest, it took a long time to see results, but we did finally. Sometimes, if she was acting like she was in a "hitting mood," I'd try to head off her outburst by encouraging her to hit the couch pillows as hard as she could--we made a game out of it, and it helped her work out her frustration about the crying.
I'm a big fan of Madelyn Swift, whom I heard speak once and then I read one of her books. She's a pain to read, but there's good advice in there. Good luck with your little one, and hang in there! I'll be asking you for advice on mine next month probably...