Sorry for the wall of text!!!
Hon..you're in a tough spot. You also did the one thing that crushes an NFP, at least, that would crush me. Trust was indeed shattered.
I dunno if you can recognize this when you look back at your relationship, but the bond that I have with my INTJ is one build on teamspirit (stronger together against the world), trust (I know you have my back) and vulnerability (I know you love me for who I am, warts and all), not to mention no judgement (a consequence of that vulnerability) and honesty (completely sharing oneself, getting naked down to your soul with him).
When I get naked with someone, I get naked to my soul. I bear all. And it scares the living daylights out of me, but it's what I consider trust and being completely *completley* honest with that person. That means that all those little nasty things that I don't like about my personality...I show you. All my weaknesses, all my insecurities. I'm betting she did that.
Imagine how it feels to have the one person in the world that you trust with this stuff (coz most people find us weird and we don't do this with them as they wouldn't understand), that you thought was your soulmate and would never judge you, would keep you safe from the world, and understand who you are, love you for who you are, slap you in the face like that. Break up with you because you apparently do not care. When you've poured yourself into a relationship with *everyhting* you've got. The man you've known to be so stable (you guys are usually our rock), to be so loving, so reliable, so safe..suddenly turns on you and basically tells you that you are failing at the very thing you value. That you aren't good enough. That he's rejecting your love. And you feel like you don't even know him. Could he be right? Did you fail at actually caring and understanding you? If so, are you then still right for each other? What are you doing being together then? If after three years, I don't know him...I must've been blind and been deluding myself. And clearly, he sees it too..he doesn't love me coz I'm unable to understand where he's at. I'm not good enough for him.
This, and more is probably what you caused when you broke up with her.
I personally bond through vulnerability. To not be good enough when I show you nervously who I am, is one thing. I know, I'm prepared some people will reject me, and it *hurts*. But to get that response after three years...when you don't even see it coming...And you're the one who's wondering if she ever loved you???
Can you see how this would lead to a looooooot of self-reflecting, self-doubt, hurt and insecurity?
You've completely turned her world upside down. If she truly is NFP, she lives by her values and feelings. To have consistent feelings is something of a marvel for an ENFP (ask around

, we always change our moods). To have them for three years for the same man...it means something. You've upset that balance. Made her question the validity of the things she's always considered to be true, and forced her to reevaluate all those things she perceived to be right, as she had no way this was coming. Somehow, she missed something, didn't know you as well as she thought she did..it means she has to go back to basics to find out what she missed. And change everything.
Then you tell her that you didn't mean *any* of it. You appologize and *tell her* it was only mean to *hurt* her. This is your defense? Really? That's driving the dagger home, imo.
So...you find it fun to play with my emotions, ruin my trust and deliberately harp on the very thing I trusted you with...my vulnerability? I get naked in front of you and all you can do is mock me? No worse, you tell me it's irrelevant that I shared this with you and just there for your enjoyment. For you to kick like a puppy. You clearly do not even recognize a treasure when you see it. Or know how to treat it properly
Where was that respect again you were talking about?
Ok, time to give you a break.
I *know* you were frustrated and you wanted to lash out. And I know you love her to death, I understand you made a huge mistake and appologized for it. I can feel the fear of losing her in your words, and I can see how much she means to you. I *know* you didn't mean to do this to her. But you did. And yes, your regret should count for something, but it might, depending on her own insecurities, just not be enough to repair the damage. Especially not coz you, in your fear of losing her, are kinda coming off like 'get over it already, I said I didn't mean it!'. That's emotional blackmail, pressure and all sorts of thing that are just going to make her go cold and confirm that you're not the man she thought you were.
I think you two would be a great couple if you could grow together. I genuinly can feel the respect and love you have for her. The question is...can she? Has she? And does she still dare to trust her perceptions?
Personally, I think it's time to sacrifice the very thing that got you into this mess and level the playing field by being vulnerable like her: Your Pride.
Realize that you may lose her. And she may end up hurting you like mad. That she could be the One that Got Away. And decide. Is she worth making this sacrifice for? Is she worth potentially risking excruciating pain becoz you made yourself emotionally vulnerable? If so, I say you write her indeed a letter, acknowledging what a moron you've been, in detail. Tell her you cannot stand the idea of a life without her. Tell her why. Most especially, tell her what motivated you to want to hurt her like that. Soul-search and acknowledge that there (I'm guessing now) were things that were frustrating you, list them, and explain how they probably influenced you and how she became the target of that rage. Give her that data, so she can finally *understand* why you did what you did. So, if she decides to come back, she has a way of recognizing when it happens again (which btw, you should work at preventing, but hey, you never know), and having a strategy to deal with it. Understanding helps us to see that the anger isn't really pointed at us, even if the words are, and that you're just in need for a vent (this is why when I start yelling at my bf, i start with an appology that it's not personal but im fed up

). End the letter by surrendering. What I mean is, tell her you'll understand whatever decision she makes. That you'd be sad beyond all reason to lose her, but you'd understand why she'd do it. Also mention that you still hold the hope that somehow you can work through this and grow as a couple from it. Don't guilttrip her, just express genuinly how you feel.
BTW...if you do write that last part, mean it.
Don't just write it, feel it. Surrender that pride. Stop protecting yourself, and going 'maybe she doesn't love me as much' and 'shouldn't she grant me the same??' That's your fear and pride talking. I have no doubt she did love you that much and struggles very hard with wanting to trust you again. Face the consequences, even though you may not agree with her decision. Accept that the ball is literally in her corner now and she has the right to choose as she sees fit. Just as you had the right to break up with her when you did. Let's hope she doesn't fall for the same trap you did, when your emotions overtook you, and sees the potential that's still there. If not..so be it.
After the letter..give her space. What she desperately needs for you is to give her the space she needs to sort this out. Sort out your own emotions at the time, don't let them drive you batty, self-reflect till you find an inner calm, till you've accepted that you are in fact powerless in this situation. You've done all you can. All you can do is accept what's coming. Be calm. It might just remind her of the man you used to be to her: her rock and safe haven.
Above all..I wish you strength. You have a rocky road in front of you. Good luck
Note: I wrote this as I perceived her to probably feel atm based on the way you described her, in an attempt to understand her actions and motivations. It's not meant to make you feel bad about yourself, or belittle your side of it, it's meant to be able to help brainstorm on how to deal with the situation. I do believe that people are entitled to mistakes and that it should be something you work through as a couple, but it's hard to sometimes see past the hurt they've inflicted and keep yourself from actually lashing out like they did, and therefore effectively making the same mistake as they did, ironically.
It was actually my INTJ that taught me how to deal with this.

He showed me how to fight without it turning personal, that fighting as a couple is more meant to resolve issues, than it is to hurt the others feelings. That would be counterproductive, as you love them. That when some things come it, it has more to do with penned up frustration than the actual person in front of you. His calm when I ranted at him taught me to do the same when he finally lost it. As I knew that that ranting person wasn't him. Something caused that within him. And he was also quick to acknowledge that, and realize he was taking it out on me, making me understand and forgive even faster. That's also why I alraedy warn and appologize before I yell at him..coz I cannot stop myself but I am aware I'm taking things out on him. He tends to just stay calm, listen and eat like an apple, waiting for me to be done ranting
