Sounds to me like you’ve Maddona/Whore-ed yourself, not your partner. What TVTropes likes to call ‘Sex is Evil, and I Am Horny.’ (TVTropes is not an approved source for psychological advice)
The really important question, which I notice you didn’t answer, is the one WhyNot asked:
If it turned out your girl was a closeted Claire Adams herself, would that solve your problem?
This may shock you, but ‘A gentleman’ most of the time, and horrible, filthy, degrading, awesome, wonderful sex (sometimes involving ropes) is exactly what a significant subset of kinky women are looking for. If you’re capable of indulging both with the same person, your problem has been reduced to finding one you can connect with. That will require some effort, (I’m still working on it myself) but it’s doable.
If you can’t, and it sounds like that might be the problem, then that’s more serious. However divine and loving you’d like to be, any relationship that involves as much lying as you’re going to be doing in this one is not going to be healthy. Because it would be, and is, lying. A lot. Constantly. That’s not being loving. Because the person she’d think she was in love with would be the person you’d deceived her into thinking you are.
<tough love time> That’s tricking someone into making a long-term, life changing commitment. Which is profoundly worse than the online porn. And if you add in cheating to ‘protect’ her from your ‘dark side,’ you’ve added being a douche on top of it. </tough love time>
There isn’t a good you and an evil you. There’s just you, with good and bad impulses. And honestly, it sounds like a lot of what you’re considering ‘bad’ is just a natural part of yourself that you’re not comfortable with. (SEX!) Nothing good ever comes of compartmentalizing your ‘bad’ impulses, all that does is allow you to deny responsibility when you want to do something you know you shouldn’t. That way lies abusing prostitutes and sex tourism. Which are also profoundly worse than online porn.
There was a time when people with ah, non-typical desires had no real choice but to do what you’re contemplating. That’s not the case anymore. And because you do have other alternatives, even if they are hard, there’s no longer any excuse for that type of deception. With freedom comes responsibility. But also (sometimes) happiness.
Talking to a shrink wouldn’t hurt, especially one with some experience in kink. Talking with some kinky people wouldn’t hurt either, but that can be hard to arrange, depending on where you are. There are often support groups for this kind of thing, and social groups as well. There are a few of us around here too, but semi-anonymous online advice/companionship is, well, just that.
Talking with your girl about it might be a good start. Don’t throw everything out all at once, but let her know you’ve been less than entirely honest with her re: things you’d like to do. She might be into some of it. (Is she Catholic? That doesn’t hurt your odds) She might not want anything to do with it. That will suck, but you won’t be lying to her anymore, and maybe you’ll go to heaven. Metaphorically speaking, that is. I’m not religious.
You may also find that once you stop lying about it all the time, you’ll hate that side of you less. Lying about what you are all the time is bad for the soul.
Additionally, (and this is important) some of the more extreme stuff you fantasize about is just that, fantasy. It doesn’t mean you really want to do it, just that you like thinking about it. Bodice-ripping romance novels, action movies, Japanese porn comics, and 90% of online fiction are all based around fantasies that no one, especially the writers, actually WANT to happen.
Kink, even the more serious stuff, isn’t bad. It’s a subset of sex, which also isn’t bad. In fact, both are fairly awesome. But if can’t respect the person you’re doing it with, odds are very good you’re going to act in a bad and hurtful way.
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I’m not sure how the ‘mental illness’ and ‘hopefully you won’t hurt her physically’ type comments are meant to help, but I don’t think they’re succeeding.