Oh dear! This is a bit of a pickle.
Like Susie said, it's hard to say without knowing your partner, but you do say that she's a bit on the conservative side. You also say that you were her guide into the sexual world, and that could be to your advantage. If she loved you and trusted you enough for that, perhaps she will be willing to follow you down another path.
First of all, let me give you one word, PATIENCE, PATIENCE, PATIENCE. Do not try to rush this or force it, in any way. That will only serve to make her more resistant. I can say from personal experience that I would rather live without sex than be forced to do something I don't want to. That explains why I was completely celibate for 8 years. I'm so glad I got over that!
I would explain to her that while you love your physical relationship the way it is, you'd like to explore some new possibilities. I would almost be inclined to turn this into a fun exercise of exploration, initimacy and trust. If she would allow you to cover her eyes with a blindfold (a bandanna is fine, and not so threatening as a blindfold), then you could gently guide her hands to where you would must like to be touched. Of course, you want to do this gradually. Even though you might want her to touch your anus, start with a few other less "taboo" spots. It's really quite amazing how different someone's touch is when they can't see what they're touching. You must also offer her the opportunity to do the same. Maybe she harbors some fantasies about where she would like to be touched as well, but just hasn't been able to vocalize her needs.
If she has a more scientific, practical turn of mind, you could always explain to her the physical reasons why certain areas are so pleasurable. When it was explained to me about all the nerve endings in the anal region, everything clicked into place. I never would have thought of pegging my partner but when I saw the pleasure he got from it, I never looked back. Although, I've looked at his backside many a time, with absolute wonderment!
That's my off the cuff idea, but I'm sure other members have other great thoughts as well.
Play On!
LadyDoc