When time outs don't work anymore...?

About a week ago, something changed between Derick and I. We have been in a power struggle! Up until then, if I had to discipline him, he'd sit on his time out step (for the most part) for three minutes. It was an effective tool for us to get him to correct his behavior. All of a sudden, he won't stay on the step. Last Wednesday, it took me 1 hour and 20 minutes of putting him back on the step for him to stay there and do his time. (Supper Nanny style) And, during that process, Mommy LOST HER COOL! It was a moment I am not proud of. I had to put myself in time out to cool off or someone was going to get hurt! This situation has happened three times in the last week and a half. Each time, was a struggle. I asked Jeremy to watch me a little closer and to give me feedback on how I am relating to Derick. I'm trying to spend more time with him one-on-one during Darla's naps; instead of doing chores. I'm trying to go to that blank place when I do have to discipline him. That place void of emotion. It's tough for me....the emotional queen. Every time I see that look in his eyes....when I KNOW he's going to test the boundary, I get Anxious! Angry! Frustrated! My confidence as a parent and mother has taken a hit. I am questioning myself now, whenever I correct him. Am I being to hard on him? Am I being too soft and getting manipulated? I know there's no right answer and each family handles it differently. I'm hanging in there and doing my best. Sometimes, it just doesn't feel like it's enough.