She is probably depressed. Who wouldn't be? It is hard to see the person you loved make a life separate from you, and to feel like you are being judged critically on a regular basis.
The lack of sex is a problem. Not only is it pleasurable, it builds confidence and allows for intimate bonding. That you withhold from her would only contribute to her apathy and depression.
What have you actively done in an effort to create a bond to her? Have you tried to engage in some activities she finds pleasurable or meaningful, or do you spend all your time indulging in your own interests, separate from her?
You say she has been a good wife; I think you owe her the benefit of making sure no stone has gone unturned in trying to save your marriage. Divorce is hard on the entire family, regardless of your children's ages, and it should not be entered into lightly.
Have you sought counseling?
As for why you feel this way, who knows? It sounds like you think because you have spent so much of your time staying in shape, you are entitled to an upgrade. For the record - being in great physical shape isn't enough for a woman, and you WILL fall apart, too. It just takes a little longer for many men because they haven't given birth.
Relax... I'll need some information first. Just the basic facts - can you show me where it hurts?