In the short term, enforcing boundaries/limiting contact as necessary (ie, by delaying returning a phone call/replying to a text/etc) is more painful. In the longer, it gets better.
I think I’ve mentioned my background to you before, but just in case I haven’t, let me summarize so you can see where I’m coming from. I have no contact with either of my parents.
My mother is an alcoholic who may have some sort of sociopathic/NPD thing going on. She has spent much of her kids’ lives doing her best to sabotage any good thing that might happen to them, she shows no remorse for doing some objectively pretty bad things (think committing identity theft with her own daughters’ information in order to commit tax fraud in their names), and though she is outwardly very religious, she uses religion only as another tool in her arsenal of manipulation. I have never, even as a small child, had a good relationship with her.
My father is extremely self-centered and has no concept of basic courtesies unless they align with his immediate whims and desires. He was emotionally and verbally abusive, and has this idea that the world was out to get him by expecting him to conform to commonly-accepted standards of practice in his field. He is almost certainly clinically depressed. I was always much closer to him than to my mother, and until we cut off contact, I would call him twice a week or so to talk and catch up.
My husband and I cut off all contact with them after some egregiously inexcusable behavior on their part connected to our wedding. (Lest I sound like Bridezilla, I should mention that their behavior then was the culmination of years of bad behavior and refusal to respect boundaries or basic social conventions.) I do not miss my mother, and I only miss the facet of my relationship with my father that was the fact that he was someone who knew me from the time I was born.
Most days, however, there is at least one moment when I feel a twinge of regret for what I’ve never had: kind, loving parents. Now that I’ve had our daughter, this has become a much bigger deal. They don’t even know she exists, and that’s as it should be. They’re dangerous. I don’t want them to know her, or know of her. But no matter how warm and welcoming my husband’s family is–and they are–they’ll never be my parents. I’ll never have a relationship where I can call my mom and tell her what DD’s been up to today, or ask if it’s normal for her to fill three diapers a day while teething, or go shopping, or any of the myriad of idyllic mother-and-daughter things that I could (and do) wish for. Likewise, I’ll never have a dad who I can call and talk to about family history, or who can take his granddaughter out to play catch.
Holidays are worse than other times, for obvious reasons.
When I was single, I dealt with this by getting out of myself as much as possible. I kept fairly busy with classes and activities. I learned (and oh Lord, was it hard!) how to start making friends, and then made a point of arranging to see them frequently. At the holidays, I’d make a list of people I knew who might like baked treats. I tried to figure out who in my circle of friends and acquaintances had little or no family, and tried to make their Christmases as special as possible with a gift (not expensive, I was a broke college student–think a hand-knitted scarf and a homemade cake, that kind of thing). I hosted little parties at my apartment. They weren’t anything fancy, just some snacks and hot cider, but I got the impression that people really enjoyed them. I’d pick someone who seemed to be having a hard time, and have them over to dinner. I also always put up a tree for both me and them to enjoy. Getting out of myself as much as I could was the best thing for me. Sitting in my apartment alone with little to do made me focus on the negatives to the exclusion of all else, and I’d pretty quickly get into a death spiral of really bad thinking if I didn’t focus outside myself more.
There were still some bad times, but I found that when they hit, if I was willing to try to bring them to Jesus and Mary that they’d ease a little. Not all the way, but enough to get through that bad spot.
Mourning what you don’t have and won’t have is important, and seems to me to be a very long and ongoing process. It’s okay to mourn. Be gentle with yourself.
Lastly, some mental distance might help you. Back then, I did have some contact with my parents. I would treat them kindly and lovingly and with the focus solely on my behavior, as though their’s didn’t matter. After a while, I got enough of a protective “shell,” if you will, that I could have a polite conversation with either of them without wanting to strike out at them verbally for their unkindnesses, whether intentional or unintentional. This also kept me from engaging too much with their dysfunction. I tried to think of it as treating them the way a saint would: showing them the love of Christ without necessarily allowing oneself to be used as a punching bag, as much for their sake as for mine.
I hope that helps. I can see that you’re in a lot of pain, and wish very much that I had a quick fix for it. God and Mary keep you safe.