Wow OP – I could have written this. A few years ago, I was you. Sorry in advance for the long post!
I’m probably going against the grain here, but I want to stick up for your partner a little. Is he generally a good partner and dad? I ask because I’ve been through the exact same experience with my DH, who lost his mum many years ago and then his dad more recently (six years ago). We met not long after his mum died. He is a really good husband and dad, but we’ve had some really difficult times because of his grief. He would always struggle around the anniversary of his mum’s death, her birthday, Christmas and other special occasions. However, when his dad died, at a completely different time of year, those dates became difficult too. Then there was his own birthday etc… eventually I noticed that he was miserable for most of the year, with maybe only three or four months when he wasn’t completely consumed by grief. I completely understand what you’re saying about special occasions. My DH couldn’t cope with Mother’s Day, but sometimes seemed to forget that I was a mum too. He struggled with Father’s Day, forgetting that he was a dad himself. Things our DD was beginning to notice and pick up on. His depression and the way he behaved was affecting us all. I started finding it really difficult to support him. We eventually had a big conversation, and I explained that while I will never fully understand what it’s like to lose both parents (particularly at a young age) he was basically pushing away the people who were still here. The family he still had, and the people who loved him and wanted to support him… and he would eventually lose us.
It took a lot but after that he went to therapy, which helped hugely, and he is now much more aware of how to manage his grief. There comes a point where you have to find a way to celebrate the lives these people lived, rather than continually revisit the circumstances of their deaths. To focus on how lucky you were to have had them in your lives in the first place. Like my DH, yours may not understood that it is okay to do this, he may feel like allowing himself to enjoy life without his parents is somehow doing them an injustice.
As someone else said, you don’t get over it, you just learn to live with it. My DH describes it now as moving the grief to the back of your heart rather than having it at the front, where it is all consuming. He and we are in a MUCH better, happier place now. I’ve not read all your replies, but if you love each other and he is ultimately a good man, I would encourage you to try to help him through this. Ask him to get therapy and maybe speak to his GP about whether antidepressants might help?
If he is unwilling to acknowledge the impact his behaviour is having on you and is unwilling to help himself, you are absolutely within your rights to walk away. Supporting someone through grief doesn’t mean you should put up with being treated badly or have to lose yourself.
Good luck!