A lot of people don't understand parental abuse. Either because their own parents were decent people. Or because they themselves grew up with abuse and think it's normal.
It took awhile for my H (then friend) to understand what was going on in my family. He was initially amazed at how upset I got about them, when they did what looked like minor things to him. Like my older sister swiping my favourite skirt and then arguing it was hers. Or my alcoholic father calling to bitch drunkenly about his latest girlfriend. In my sister's case, she had a long history of invasiveness and acting as though me and my possessions were hers. In my father's case, he had long tried to parentalise me and drag me into his endless carousel of women problems. This was a serial cheater who left my mother when she nearly died of a heart attack, after which he went on to marry 5 more times. I was 30, and fucking sick to my back teeth of their toxicity and all their drama and all the trouble they created for me.
I told H that if we were to stay friends, he didn't have to understand what was going on between me and my family, but he did have to respect that they were a major problem for me, and that if he couldnt be a support for me, he should stay completely out of it. I would not have tolerated him being friendly with them behind my back, if he had, I would have resolutely ditched him like I had ditched others who got sucked in by the charm of my sister and father. There was ZERO room in my life for people who would help my sister or father breach the boundaries I had built with them to keep me sane and safe.
Over time, their toxicity became inescapably plain to H, especially after we got together, and he now views my sister as an inordinately dangerous and criminally inclined person and my father as the most selfish destructive person he'd ever met.
Op, you need to set the same boundary with your H that I did with mine. He doesn't need to understand why you have problems with your parents, but he does have to respect that these problems are longstanding and painful for you, and that he has to stay out of them.