Dredging up a lifetime of shit? Why now?

Hi, everyone! I've been lurking around this sub and others like it for a while now, trying to find answers to what's happened to me basically my whole life, but now it's time to post.
TL;DR -- I'm 25, female. After a ~3 year relationship with a guy who scored the unholy trinity of narc+psychopath+sadist, and the first 19 years of my life living with Ndad and Emom, I am finally in a loving, awesome relationship with a man whom I love to death and back. And I'm miserable. My entire past trauma has come back to haunt me and it's eating me alive. Am I losing it? Is this a common thing? Why now? What can I do?

I have a background of dealing with narcissists pretty much as far back as I can remember. I am the only child of an Ndad and an Emom, whose favourite method of...child rearing together was copious amounts of shame and guilt. My guilt button is through-the-roof sensitive and oh so easily triggered. More often than not, it paralyzes me. So, that's the first 19 years of my life living under a narcissist's belt.
After moving away from home, I had an extremely abusive relationship with a guy who scored the sweet trifecta of narc+psychopath+sadist. It ended after almost 3 years, and I was spent. I felt old at 22. I just wanted to lie down and sleep forever.
A long period followed -- ~10 months -- in which I thought I was healing. I felt like I had recovered my life and my youth. I was dating someone utterly inconsequential, but who was good to me (most of the time) and was pleasant to have around. I thought I was finally and irrevocably free of my Nex and ALL of the damage he'd done.
THEN I met my current SO, whom I love to death and back, and finally I am in a healthy, beautiful, loving, mutual, non-head-fucking relationship. We're living together, and planning an entire life together, happy that we've found each other.
Aaaaaand I'm generally miserable. It feels like ALL my past trauma has finally caught up to me. Every childhood defense mechanism, every internalized parental voice, every hurt I've ever had, the ~3 years of hell with the narc -- it's all reopened inside of me like some twisted flood gates from hell and I feel like a walking wound. It's taking away my joy for life, my youth, again, any proverbial spark, my voice. I feel empty and devoid of emotion, even cracking a smile is too much effort. I lack motivation for even basic things, which is terrible, as I am just about to finish my Master's degree. It's harder and harder to find words and communicate effectively in daily life. I feel silenced and like I'm suffocating. I'm having flashbacks from various traumatic experiences from childhood. Recurring patterns of behaviour as when living around my dysfunctional family. Nightmares about my Nex -- had one just last night. This has all been going on for at least a year, at varying degrees of intensity, roughly since I moved across the continent to live with my SO.
It's almost as though now that I've finally found what I was looking for, for so long, now that I'm finally happy (or at least have every reason to be), my brain can't deal with it and is dredging up every horrible thing from the past. Or worse, I never healed from any of this, and have to do everything over again. It's taking a toll on my current relationship. I don't want to bury my wonderful, loving boyfriend in this torrent of hurt. He doesn't deserve this.
So...why now? Is this a common thing with people who've gone through prolonged narcissistic abuse? Am I losing it? What should I do?
P.S. To whoever made it this far down my Great Wall of Rant, you have the patience of a saint! Thank you, and any advice or ideas are so welcome.