Boy, you really screwed the pooch on this one, didn't you?
Don't worry, the world doesn't have to end, and neither does your relationship... but it's going to take some work on your part.
Your girlfriend knows you've got a kink. She knows that, given your druthers, you'd be having some pretty rough sex. And she knows that you've got a mental block about having that rough sex with her. Like it or not, she's perceiving that as rejection, and probably has for some time.
You poured fuel on that fire by telling her that the best sex you ever had was rough sex with some 19 year old. In other words, you've basically told her that not only was the best sex you ever had with someone else, but that sex with her can NEVER be better than that because you can't get over your mental hangup enough to HAVE that kind of sex with her.
What's the solution? Get over the mental block. And that's where the work comes in.
First off, you seem to equate rough sex with a lack of respect. Thousands of happily married kinksters would beg to disagree with you. Ok, well, the Bottoms might beg... the Tops would rather beat you for presuming they don't respect their Bottoms ;) My husband and I certainly disagree, but since we Switch sexually, we'll both beg AND beat. LOL
Seriously though, I think you need to do some reading up on loving BDSM relationships. I think you need to be able to read about examples of happy couples who have been able to incorporate all sorts of kinks into their relationships. And I think you need to realize that one of the forms of respect you can lovingly show a partner is to give them what they want, sexually.
And guess what your girlfriend wants, sexually?
Yeah. That.
You know, you don't have to do "rape roleplay" to have nice, rough sex. It can just be part of making love. You don't have to talk or act out a role. It doesn't take a roleplay to smack her ass while you penetrate her from behind, or to put your hands around her throat (AFTER educating yourself on asphyxiation play!!!!), or to slap her cheeks (AFTER educating yourself about proper face slapping). It doesn't even take a roleplay to tug her ear close to you by grabbing her hair at the nape and whispering, "are you my little slut/fucktoy/whatever?" while you boff her brains out.
What it takes is acceptance that kinks are cool and fun and a good time when shared by a couple who shares an enjoyment of those kinks. I can tell you what would happen if you ever even IMPLIED to my husband that he somehow doesn't respect me because we had a bit of "slap & tickle" in the bedroom last night - he'd laugh in your face.
And so would I.
So get your fingers a'Googling, and educate yourself about your kink and how couples incorporate it into loving, RESPECTFUL relationships and marriages.
That's how you start to fix the mess you made... by allowing HER to give you "the best sex you've ever had."