@Magicmegan firstly, I'm glad you've told people this. It's not an irrational fear, its trauma that has shaped you. Definitely sounds like ptsd/c-ptsd symptoms. You have my sympathy.
Secondly, he wasn't ever going to slit your throat, he was trying to get you both back under his control. He wanted to terrify your mum, so she'd never leave out of fear. It would be rarer than unicorn shit that a murderer would give you a heads up first.
My one threatened all kinds of mad shit but only ever actually almost killed my mum through strangulation, which was done with temper, the threatening to kill her with knives etc he never once pulled a knife out. Just said it.
I also had a very intense childhood, absolutely entrenched with domestic abuse from my sperm donor also (you probably guessed from what I said)
It fucked up my teens and 20s up if I'm honest. I did go for EMDR but it didn't "agree" with me, so please discuss with your therapist the possibility of unlocking more memories that may be hidden for a reason. (It uncovered sexual abuse I suspected occurred but I couldn't remember specifically, to the point now I remember it very in depth, so whilst for many people it's absolutely incredible, please also be aware it's opening Pandoras box) but please, do seek counselling in general.
You have been left with so much to process, do you have survivors guilt? I believe it's common with kids growing up in households of violence, where we see our mums absolutely destroyed, so it kind of haunts us that it wasn't as bad for us, even if it was, which sounds nonsensical.
What he was, was a pathetic little person who wouldn't pick on someone his own size. He's a bully, a coward and he won't bother with you now. I'd bet my last fiver he only wanted to bother to freak out your mum.
He will have found someone else to terror or maybe he's taken to some type of substance abuse. (Mine drank himself to death) but your DM got you out and kept you safe. She set a boundary and she was strong, he'll know from your boundary mum has raised a kid that nobody fucks with and he's wasting his time.
Your life doesn't have to be beholden to this absolute fucking bastard and your fear will improve once he's dead, but it kind of doesn't empower you as much as you hope it will.
You have to learn techniques mentally to cope with fighting the intrusive thoughts he will harm you.
I want you to live a happy life, you've been through some serious shit and it's been hell for you, I know, but you can't allow this fear, even though its not illogical, you've been scared of this for so long, it's not ingrained, but trust me from someone a bit older, get a handle on it now.
I went wild, lots of drink and drugs, lots of behaviour that I'm both ashamed of and proud I didn't die from for many years.
I needed to talk to therapists, not drug dealers, I needed to meditate, not drink, I needed to give myself affirmations and be nicer to myself, not let every Tom, dick and Harry shag me for approval.
Your mother kept you safe, she was strong and didn't just go back out of fear. You have her spirit in you, just because she was there for a bit, she got out which is remarkable. I'm proud of you both.
If you ever need a chat, please inbox me. I'm always around.