When my DS (4.5) and I get into this it becomes a cycle - I am annoyed and try to control more, so he does more things that upset me, so I get even more awful. It sucks.
During these phases ( which were worst about 3 mos after my DD was born, but still come and go) I return to "survival" mode. There is only ONE goal. Make it through the day without yelling at anyone. Nothing else matters, not the dishes, not the errands, nothing.
When my son is doing something that is driving me crazy I ask myself, "Is this dangerous/destructive or merely really annoying?" If it is just annoying, as most things are, I just let it go.
I also think about "is this the hill I want to die on?" Meaning is whatever this is about important enough to be my ultimate battle?
I have also found that in that moment, if instead of grabbing or yelling, I blurt out "I love you!" it really changes the energy of the situation. He is surprised and then happy, and really you can't say it without having the emotion of it sneak up on you.
Finally, when I was first adjusting to having two, I had to have a plan for what to do when it was spiraling towards BAD and my son was being physically aggressive. So in a moment of calm I decided that our safety was above all the most important thing. And when he got nuts I would say very calmly "You need to go sit in the stroller" which was parked in the living room. Having a plan in place meant that I didn't need to loose control - I knew what to do! And the moment of physical separation gave us all safety and a moment to regroup. I think that sometimes our mama bear is working so hard to protect the new little one that she snaps at the older annoying one.
Lastly if nothing else works I leave the house with the kids to go somewhere. I get them both strapped into the car and then I stand outside the car and take a few deep breaths. I enjoy my 2 seconds of peace and control, before I get in to deal. Besides, even on my worst day I am much nicer in public.