Kendra,
Although most of us who are transgendered and cross-dressers totally understand why you like to dress, and why you have kept it a secret from so many people for so many years, it's quite likely that you wife will understand none of it.
What's worse, if you have been dressing around the house, hiding your wardrobe, and hoping she wouldn't find it, or the activity on the computer, then she may already know and is afraid to let you know that she knows for fear that it might hurt you.
The reactions to the formal revelation are similar to the reactions to learning that a loved one has died. Shock, anger, sadness, acceptance, and resolution. And it's not a linear progression either. She may seem accepting one day, then angry about it another day. She may also see it as a potential bargaining chip.
The degree to which she will experience your revelation as a death of the man she knew and loved, will be largely a factor of who you have been prior to this revelation. If you are "Mr Macho" at home and in your social life, and what she loves about you is that your strong, masculine, aggressive, and a good "protector", then she might feel like everything about you is a lie, that the man she knew was an illusion, and she might even hate your femme self.
On the other hand, if you have always been kind, patient, gentle, supportive, loving, and "feminine", she may have been waiting for you to come out, and might have even been afraid to let you know that she was attracted to you because you were so feminine.
Simply put, the more you've lied, the harder it's going to be to make amends for all the lies. For a woman, it's not the cross-dressing that's the issue, it's the dishonesty, the betrayal.
You need to be ready to support her in any way she needs you through all of this. If you have never told any other living person, face-to-face, about your desires, then you need to tell her that. If you have tried to tell others and it has gone badly, you need to tell her that. You need to let her know how vulnerable you are, and how much you love her and how much you don't want to lose her. She needs to know that you love HER, and find HER sexy, beautiful, attractive, and wonderful in every way. She also needs to know that this is not a secret you kept lightly.
Next, you need to let her begin to set the terms. How does she want to deal with it? If you're lucky, she will be into it, maybe even have some bisexual tendencies and fantasies, and will want you to make love to her as a woman. On the other hand, she may insist that you not do it at home, around the kids, or anywhere else where others might find out. She might suggest that you go to a cheap motel and dress up and stay in the room. She might want to dress you up evey night or all week-end, or she might want you to limit yourself to once a month at the motel.
But she may also have some other demands. She might want you to stop smoking, start losing weight, quit drinking, or do more housework like cleaning the house and doing the laundry - especially if you've avoided the "women's work" in the past. Even if she says she wants you to scrub the toilets, you should seriously consider ANYTHING she suggests as not a "It would be nice if..." but as something as important to her as your cross-dressing is to you.
The one thing that you can count in that she will very quickly be aware of how important this is to you. It was important enough to keep it a secret for all these years, and important enough that you were too afraid to even tell her for fear of losing her. It's only natural that she will want to use that leverage in ways you may not be prepared for, and can't predict. You can probably already guess what it is she will want, but more than likely there will be some twist to make it more interesting.
If you listen, really listen, for what SHE really wants and needs, there is a good chance that you will work out a solution that works well for both of you. She may even moderate her demands. You may end up having a much closer relationship as a result. One based on honesty, integrity, and authenticity and mutual trust.