It sounds like your holding back is an unconscious defense mechanism. You are conscious of the existence of the defense mechanism, which is good, since many people aren't even aware of the existence of the defense mechanisms that cripple them. In order to soften the defense, you need to first understand why it's here.
The unconscious portion of the psyche creates defense mechanisms in order to protect the self from danger. Unfortunately, defense mechanisms really don't work very well and they often bring about the very thing that you are trying to protect yourself from. For example, think of a person who bites his tongue when he's angry because he's afraid of making his wife angry, and guess what, his silence makes her angry! In order to understand what your unconscious mind is trying to protect you from, ask yourself, 'What do I think would happen if I Ëœgive it all up' to my wife?'
Sit back and let any and all your fears surface. Be prepared to be surprised when you discover what you are protecting yourself from. You may feel afraid to love and to lose. A lot of people are afraid to let themselves go, physically or emotionally, because they think that they will suffer terribly if and when their beloved leaves them either through abandonment, divorce or death.
Other possibilities: you may be afraid to open up your weak, dependent, vulnerable side; or you may be afraid that if you let your guard down that your self will be taken over or swallowed up. Last but not least, you may be afraid to appear foolish or be laughed at if you reveal your true self. Once you identify the reason behind the fear of letting yourself go, then see if you can find the origin of the fear. To do this, scan your emotionally significant childhood memories and experiences. Recall when your worst fear did come true. Were you actually abandoned? Were you taken over by a bosy or controlling parent? Were you laughed at when you revealed yourself?
When you understand where the fear comes from, then start talking to yourself. Remind yourself what is different about your wife and the person or person(s) who damaged you before. This should help you separate past and present and make you feel safer to open up to her. I think you also might want to try to open up to her in stages. You might start by talking out of the bedroom. I am thinking that opening up while having intercourse may be too intense and overwhelming. You might want to ease into the opening up by telling her one thing that you feel like revealing.
When you see that she receives your revelation and that your world doesn't fall apart, then you tell her some other intimate piece of information. I think that the opening up to her while you're having sex will come naturally--no pun intended--as you open up in other ways. Don't try to force yourself. Let the self exposure unfold naturally and in its own pace. Let me know how you do.