Well, a lack of sex can certainly be contributing to how you are feeling, I would think. Have you tried talking to your wife about this? I find it hard to place myself in the mindset of someone your age and of the opposite gender, but perhaps her sex drive has decreased because she no longer finds herself attractive? I know that it is usually natural with age as hormones drop off (at least for men), but I don't know much about the psychological aspect of it.

Fantasies are fine, but I would definitely not act on them. That's not fair to your wife (though granted, if she knows that you're still a sexual being, I don't think it's entirely fair either for her to refuse to be with you, but two wrongs don't make a right).

This is my input, at any rate. I hope I was of some help.