Apologies,
Arius, for the following (likely long-winded

) replies, but my gut says that you are totally sincere, & have been "kept in a box" for so long that maybe you need as much encouragement as possible to properly consider your situation.
(As I type, my mind keeps playing "Scarborough Fair," telling me she's setting you one impossible task after another, "THEN you'll be a true love of mine.")
Let's begin with basic issues of trust, & whether someone is worthy --
I explicitly expressed that I would not be comfortable with her having a 24/7 dynamic with someone else
There you go: "asked, & answered," as they say in the courts.
one of many things I plan to discuss once I have an emotionally safe place to live.
By all means, yes!!
Please care for yourself first, else you're no good to anyone else.
However, I cannot be optimistic about any discussion. As things stand, there's no clear motivation for her to respond; after you separate, the only reason I can discern to speak candidly with you is if she wants to drag you back into a situation that's not much different from what you now have.
she was saving this better version of herself for someone worthier.
No. Just, no.
Even if that sort of thinking had ANY truth, that'd clearly say that
she has been LYING to you all along. How is that YOUR fault?
(Let's say that she was totally honest with you up to a year ago when she decided to be a slavegirl, & her "master" has been using her to mess with you since. That'd mean she's intentionally abusing you NOW. So, she's either
currently abusing you or has been abusing you all along.)
Who's to say that this "better version" isn't just D/s play-acting?
But you MUST cease such negative comparisons, OK?? Even if you're some sort of grossly disfigured Phantom, she CHOSE to be with you. It is NOT your fault for HER choices. And if she now chooses to make you a secondary (tertiary?) part of her life, the "old contract" is over anyway.
I'm wondering if I'll be more confident once I'm on my own.
IME? Probably!! Sure, it might take months, to wean yourself from the day-to-day mind control, but when that crap starts to fade it's only YOU being subjective that will prevent YOU from seeing
objectively that you are an introspective person AND capable of outward understanding.
I actually think I'm pretty great. I'm just very aware that almost nobody else shares that opinion.
Well, there ya go, to hell with THEM then, right?

Go forth & be thine own
true self. (Gods know, I ain't pretty, but calm honesty has earned me MUCH support.)
I'm not under-selling myself.
Actually, you have been in these threads. That can change.
Beauty privilege is real, and I don't have it.
Neither do I. But my "wake-up call" came when I was out with Julee on our first date. See, she's a 5'9 auburn-haired Irish flight attendant, & I'm this skinny nerd with a 156 IQ. An hour in, ordering our second coffees, she paused, sat back, sorta looked puzzled, & said "I never thought someone like you would be interested in someone like me." My immediate reaction was, "that's MY line!!"

She wasn't at all used to anyone who got past her stunning looks & found her to be smart, witty, & challenging.
If you don't believe in yourself, nobody else will either. Past that, whether you'll win any beauty contests really is immaterial.
love is rare. It just is.
Well...
unusual, maybe, but not rare. Largely, it's about as "impossible" as YOU want to make it.
I'm angry at myself for not being as awesome as he is
Well, heck, there ya go: HE AIN'T.

You're comparing yourself to an
ideal, NOT to another human. Let her devote herself to being his play-toy & actually trying to fulfill the "24/7" myth.
I think I'm at a place where I can accept spending my one life alone.
It's a good place to be AND that phrasing is absolute unvarnished bullshit. (Hey, BTDT.

) Finding your personal "absolute zero" DOES NOT mean resigining yourself to be forever stuck there. If you WANT to stay alone, that's cool (I'm enjoying a "hermit" phase myself), but IMO you first very much do need time & a safe space in which to
rediscover your worth, without it being undercut every day/hour/minute.
It felt shitty to always be rescuing her, and it feels shitty to withdraw support from her.
There ya go: classic enabler thinking.
Don't accept martyrdom. Unless you feel in your heart that she is weak &/or stupid, you'll be able to admit she doesn't NEED a rescuer at all; if she's that dysfunctional, then fixing her is likely far beyond your skills.
I broke myself of the habit by realizing I had two choices: keep supporting my lover so that she could keep making crazed attempts to hurt herself -- & maybe the next time would be worse, or even permanent -- or
be responsibly selfish for once: I admitted the toll her addictions were taking on me, & accepted that if I didn't start taking care of myself, I'd soon enough be useless not only to her but to everyone in my life. I insisted that she needed professional help; she got angry, broke off the relationship, & we never spoke again. Yah, that path hurt, but probably MUCH less than if I'd clung to her for the rest of the ride.
She's not a drama queen. ...she was trying to make me feel included a little in their dynamic.
No; nonsense. Either YOU are "drama queening" in relating the story, or SHE is being accurately portrayed as a scenery-chewer.
FWIW, unless the collar is entirely steel cable, any EMT will snip through it in a fraction of a second. The hasp is immaterial.
And if the "second key" thought is her proposed way of making YOU feel somehow important, I have to call BS.
If he wants that position of dominance over me, he'll have to fight me for it.
The "fight" has already happened. You lost. There will be no rematch allowed. He's in control until she gets tired of it; even if she quits, there's no guarantee she won't just run the script again with a new "master."
she does tell me she loves me multiple times a day. She is sweet and affectionate most of the time and has a huge heart. ... I don't doubt that she loves me.
There's a model of child abuse I once studied, very particular to the mother. In it, a woman will "bottle up" her frustration, doubt, & anger, outwardly appearing only to be a little hurried. At some point, there's just one straw too many. That's when the hitting starts. The literature is full of truly horrific accounts of a woman beating or scalding or burning her baby. And when that peak anger passes, the woman is
instantly remorseful, & makes heroic efforts to save the child. It's almost a two-personality situation: there is ZERO transition between the two states, rather like flipping a switch.
So I don't doubt that the same person who is messing with you actually does also harbor sincere warmth for you. However, love doesn't justify abuse.
she was neglected as a child and doesn't know how to show love.
YANAT --
you are not a therapist. She's working to fix her scars, or she's not. Either way, it's not YOUR problem. And giving her a "golden ticket" to be a jerk whenever she pleases is NOT a way for her to get better.
It's not great that you wrap up on an enabler note --
I don't want to lose her. I can't describe the love I have for her despite all of her flaws. She understands and accepts me in a way that nobody ever has.
See, that's talking in circles. If she truly "understands" you, then she couldn't possibly avoid understanding the pain she's inflicting on you. If she's okay with hurting you, then your underlying assumptions are clearly flawed. Setting aside the matter of her
potential, her actions are at best questionable.
It's high time you set aside the root assumption that she's somehow "the only one." How often have you allowed anyone else to get close enough to maybe show that your partner is far from perfect?
In any case, YOU need to do some healing & regain inner strength.
Do not backslide. You first need to "be the best you."