I feel for you buddy
This must be hell to go through. But I'm going to add my two cents to everyone else's. Obviously, I don't know you, and I'm not an expert, and ...(caveat caveat caveat). But here's my thoughts.
1) I agree with others who've said that this is primarily a trust thing. If you'd revealed your name was in fact "Harold Bananahammock" after she'd spent years thinking it wasn't, she'd have the same kind of problem - the "he's not who i thought he is", or more accurately "he's not who he told me he was" problem. It's a big step, a big commitment, to decide to marry somebody. Something like this can really make a person seriously reappraise it. There's also the element that this particular secret has a sexual aspect - what she's found effectively is a porn stash, and that opens up the "aren't i enough for him" can of worms. There are lots of couples out there that use porn happily together (personal experience here) but if you haven't been then from her viewpoint when she found out she was hit with a double whammy of (i) you're into stuff she had no idea about, (ii) you indulge that with images of *other women*. Not knowing her, I'm not sure which of the two stung more, but they both stung. All stuff you know already and are already beating yourself up about, I know. But -
2) Abandoning the internet and a lifelong interest in feet and tickling probably isn't the answer, for a couple of reasons off the top of my head. One being that as I've already said her problem isn't really with what you're *doing* it's with what it represents. There may be avenues to involve her in this side of your life as you do others, make her central to it, show her that this doesn't in the slightest reduce your love and respect for her. It's like, okay, I have a foot fetish (not uncommon for this community) but I don't leave partners who play that way with the impression they could just cut themselves off below the ankle and leave the rest with me. It's something I get a great deal of pleasure from, but not something that gets in the way of the fact that primarily what I get pleasure out of is having this amazing woman sharing this intimacy with me. So, again echoing other people, talk to her about it. In fact, ask her about it. I'll come back to that in a minute.
The other reason I don't think abandoning the internet is the answer is because you have a sexual compulsion to seek out these stimuli. There's absolutely nothing wrong or shameful about that as far as it goes; but if you cut off one avenue in the internet and you don't open up an avenue with your fiancee, you *will* find another outlet. You will find a magazine, or an internet cafe, or start hanging around with a cameraphone taking pictures of girls kicking their shoes off in the park, because it's habit forming and you can find a hundred ways to rationalize it. And that will be curtains for your relationship, when, inevitably, she finds out about it. I say 'inevitably' because I have a suspicion that you weren't "stupid" to let her find out about this, on some level you wanted to resolve the conflict between a woman you love and an activity you love being separate for you. You wanted to bring them together, and actually you've given yourself an opportunity to do that.
3) With regards to discussing this with your fiancee. The temptation in this situation is to really go overboard on sackcloth and ashes, as a mark of how much you feel for her. This is a dangerous temptation to give in to, for a few reasons. Firstly, it seriously alters the dynamic in your relationship, and especially if you've been together a while that's very tough to deal with. You'll be forever playing catch-up for this terrible wrong you've done her, and neither of you will allow it to be resolved. It will fester and you'll end up resenting her, or she'll end up losing respect for you. Either way spells trouble. Secondly, you acting that way vindicates her immediate impression that you don't trust her and you do need something outside the relationship to satisfy you. Take a long hard clear look at yourself at this point and make sure that you can emphatically deny both of those are true. If you really don't trust her, or you really aren't satisfied with her, it's cruel and manipulative to lie now and make her believe otherwise; and again, you will only end up 'betraying' and hurting her again in future if that's the case.
Assuming it's not true (I personally don't think it is; I'm pretty sure from what you've said that you really do love her) you need to try to show her it's not true. Offering to give it up for her just proves that you've spent all this time giving her up for it, which again hopefully isn't a fair reflection of your relationship so far. I suspect that part of the thrill, maybe a very large part of the thrill, is the fact that it's something secret, something embarassing, something shameful. As a sexually submissive foot and tickle fetishist, I dig that completely. But just as there's a world of difference between reading about tickle torture and experiencing it, so there's a world of difference between fantasising about doing something secretive and shameful, and actually doing something in secret you ought to be ashamed of. If you're serious about her, and I think you are, she deserves to know who you are - all of who you are. And you then need to deal with what she thinks about that. Right now, what's most important is how she feels and what she knows. You need that information so you can see what chance there is of putting things right. It may be that you have hurt her too much; it may be that she's going to need time. But if you assume these things without finding out from her for sure, you can believe it or not make things even worse.
Relationships always come down to communication. Honesty and openness are good and healthy. If the relationship's strong, it can get over the most incredible things. I'm not proud to say I've both cheated and been cheated on in the past, and I've had relationships that survived that and relationships that didn't. The key for you right now is to concentrate on learning what kind of relationship you actually have with your fiancee, rather than trying to manipulate it into the kind of relationship you think it should be. Do that openly, honestly, lovingly, respectfully, and you may find the two things are closer than you dared hope.
Good luck.