I'm glad, DearTeddy, that you're seeking counselling for the abuse which you endured as a child/teenager. Your determination to break that perpetual cycle, so that your own children never have to live their lives in fear of their parents is commendable.
I could describe the abuse - physical, emotional - which I endured growing up, and the abuse - verbal, emotional - which I still put up with (to a certain degree) from my parents, my mother particularly, but I won't. What I will say, however, is that it was counselling, and speaking to former friends of my parents (who did nothing, or precious little to help protect me as I was growing up - and who now all claim to regret not stepping in but "it wasn't the done thing back then to get involved" 
), which helped me to piece together that both of my parents had been abused by their parents. My mother, particularly. They simply repeated the abuse. And I was determined to put an end to it, for my own children's sakes.
Having said that, if my child had been smacked hard enough to leave a handprint, had their face pinched and have their grandparent hissing who-knows-what (and I do wonder if it was along the lines of "don't tell Mummy what we did", actually), then I would have reacted as instinctively as you did, OP. Your slapping of your mother... no, perhaps not the best of actions, but I can understand why you did it. And I understand that your remorse is tied up, still, with the residual fear of your childhood - somewhere inside of you, I'm willing to bet, you're waiting for her to beat you in response.
Actually, I have slapped my own mother. She hit me when I stepped between her and my then-small daughter, I forget why, and I slapped her in self-defence. I also told her that if she ever laid a finger on me again, I would do more than just slap her. She's spent the last 20 years restricting herself to verbal and emotional abuse towards me, but at least I no longer instinctively flinch whenever she lifts her arm.
My son (14) and I are VLC with my parents now, for various reasons. My 23 year old daughter is adored and feted by them (although I'm waiting for them to turn on her, too, now she's refusing to allow them to control her as much as they previously have done). My brothers went NC years ago (to the point where she only found out DB2 emigrated two years ago, last week!). It can be done.
Just as breaking the cycle to protect your children from abuse learned at the hands of your parents, can be done.
Good luck.