It's easy to go back and obsess over small details of your childhood - it's a form of self-blaming; that whole "why didn't I see it sooner?" thing. It is healthiest to let that go - not "forgive and forget", by any stretch of the imagination, but to accept that what happened to you in your childhood was wrong and focus on living in the aftermath.
I was born in the 90s and I'd say about half my friends (including me) were smacked as children. The majority of my friends now have great relationships with their parents, because their parents were doing what had been done to them and were misguided but otherwise good, stable people and good parents who were just wrong on the smacking front.
I have no relationship with my parents, because their behaviour from childhood through to now isn't conducive to a healthy relationship. They're two toxic people who realistically had no business getting together and having children, and their toxicity has continued into their childrens' adulthood and started to affect their grandchildren. That's the point where I said enough is enough and cut contact.
Rather than dwell on these things from childhood, it's ok to tell yourself - "these things were wrong, I didn't deserve to have them happen to me, and they were wrong to do it". Then, separately, you can consider your relationship with them now and whether that is something you want to continue.
There are lots of flying monkey types on MN who'll tell you to keep contact and be horrified that anyone could cut off their parents; that's ok - they're limited by their experiences and it's not something that's easy to imagine until you're actually there.