Hi! Most of us have been in your shoes before. I was terrorized by my son when he was 2-4 years old. I really recommend the book they already suggested "The Explosive Child." I had taken my child to 2 psychologists who, by the way, gave me completely opposite advice, and it had done no good. When I read the book, I noticed a huge difference in my son after 24 hours of implementing the advice in the book. And it wasn't just me--his preschool teachers said he had his best day ever just a day after I began using the book, and I hadn't even told them I was going to do anything different! Not everyone may have such quick results, but I began liking my child and my life again. Don't despair--things can get better.
PS. Hi to everyone else! It's been a long time since I popped in here. My difficult child is 6, almost 7 now, and I guess I'm moving up out of the under 5 forum.