There are many couples that end up in this situation. If your wife is withholding sex, there are many things you can do to improve the situation.
First, men and women are very different, and need different things from a relationship. While men can often happily have sex regardless of how they are feeling, women need to be in the right mood or frame of mind.
It can be difficult to feel sexy or want to have sex if you are depressed, stressed or generally upset.
So my first recommendation is to try and make your wife feel relaxed. Maybe take her to a spa break or just run her a candle lit bath.
The other big thing is foreplay.
So many men seem too eager to jump into bed and get it over with. Someone women are like that too, but many like to take it slow and enjoy the build up. Have you tried taking things slower by kissing and touching first?
What about your relationship outside the bedroom? How is that? Are you spending enough time together? Have you grown apart or spend all day in your own worlds. People have to be close to be intimate, so try working in your relationship.
If none of these work, you may need to try counseling and sex therapy. Many people look down on this or are ashamed of having to consult someone about sex, but really it could be the best thing you ever ever do