I had a terrible relationship with both of my parents all throughout my childhood. As an adult, I've rebuilt a decent relationship with them, though we are not close and they are the often the last people I share personal information with (and certainly not people I ask for advice or input, ever - that's a bright-line hard boundary). I do, today, call both of them up just to talk, and they've learned where the boundaries are and respect them (because they respect me).
They were ridiculously immature when they had me - I only sort of got confirmation of what I'd suspected for a long, long time: it was a shotgun pregnancy wedding. I hadn't realized just how much their families had rejected them - my grandmother refused to even see me as a baby apparently. So yeah, came into not the best situation.
My parents did a lot of shit wrong - most of their dumb moves and mistakes hurt them, mostly, but a few left me with a lot of shit to get over, and a profound sense of being unloved and unwanted. I got the fuck away from them as young as I possibly could.
HOWEVER....I also came to see I'm not a special snowflake in this regard. There are a ton of people just like me, and my parents were neither evil nor malicious - most of their mistakes were just that and completely unintentional - they were often just a 'cycle of child-rearing' where they dished out what they'd gotten themselves. They also kept me fed, clothed and schooled. Both my parents were cultured and well educated and they provided me with that. This is a shortenend list, but I didn't lack for a lot of the basics of a good childhood.
I met other people, with truly fucked up families, and truly awful parents (for example, the father of one of my friends was molesting his daughter - my friend's sister - got her pregnant 2x). I dated a woman who had grown up neglected and abused (half-starved, broken bones) by a truly mentally ill mother. My parents were fantastic saints and very very decent human beings by contrast.
I'm not sure everyone can have a relationship with their parents - some people are just so toxic the only answer is to get them out of your life. But for me, I've learned that my problems are my own to fix - blaming them on my parents does no good, and that with effective and clear boundaries, I can have a nice relationship with them. I learned to forgive the past (really, pretty easy) and to set the tone for how things are gonna be in the future.
My sister was the first in our generation to have kids, and I watched my mother pull the same obnoxious undermining crap on my sister with my nieces as she pulled on my dad when we were kids, but my sister didn't set the appropriate boundaries and then enforce them. I pointed this out to my wife very clearly before we had kids and pointed out the tricks my mom gets up to - not to slam my mother, but to explain why my "love" for my mother came with it's own set of "conditions" (boundaries) - and how important it was that we maintain a unified front there as well as with our kids. After she observed mom's stunts a time or two first hand, she believed me, and when it came to our kids, she backed me 100% on setting the boundaries.
The bonus of this is, my kids get to have a relationship with grandma - and grandma has some great things to offer - she's not some toxic evil witch. My kids love grandma and grandma gets what she wants. And it's ok that it doesn't involve me telling grandma "I love you" or being super-close.
I would advise people to forget (if not forgive) past wrongs so long as the patterns are ongoing.
My wife, OTOH, has a great relationship with her parents, and they are lovely people. Oddly, you don't need to enforce boundaries or appropriateness with people who already have them. How refreshing!