Defiant 2 year old

Our son likes to hit me, daddy, and Mimi. He is 24 months old and has been doing this for awhile now. Some days are great, fine and other days I just want to cry because I don't know what to do. He seems to hit when he is frustrated, when we tell him no on something he wants to do and sometimes just out of the blue. I could be sitting with him doing an activity and he'll just say "Hit Mommy" and then hit me. The past three days it has been non-stop and I am just so tired of it. We've tried time outs, we've tried giving him more positive talk, we've tried teaching him to express his anger. They all seem to work some of the time but it's getting so frustrating. I find myself stealing a minute away by myself to cry because I feel like a failure. A few times I have let my anger get the better of me and yelled at him and then I would cry. I don't know what to do.