M.D.
What I have learned is that a. All mothers feel this way sometimes...this is a tough job!!! and b. this type of defiance is very age appropriate and, sigh, a good sign developmentally.
The best advice in this thread is to be consistant. That doesn't necessarily mean that you only ever utilize one strategy, but that the strategies that you use are employed for the same reasons each time. My son gets time out for doing things that he knows are wrong, such as hitting. If the infraction is, for example, a dispute with another about sharing a toy, take the toy away...it is best if the consequence matches the infraction.