I agree that bitterness is a poison you drink and expect the other person to die from. It’s not good for me to hold onto.
That said, I have an ongoing relationship with mom and the bitter feelings (about my childhood) that come up when she’s around don’t feel nice.
She’s proven in some ways that she wasn’t a great parent. It may mean she doesn’t get to be around my kiddo as much.
I’m wondering how you handled it as a parent when first encountering these feelings. Did you just decide to forgive and move forward with grandpa and grandma in the picture? Did you let us see grandma and grandpa less than you would have if you’d have had a better relationship with them?