Hiya Julie. Some old school would say if you always had these feelings, you should have been honest up front, or not got married.
Not that easy, some people supress their urges, give up their own life for the other person. When I say its not that easy, it's a need. not some fetish. kinky part time hobby. It's an overwhelming desire to be the person you need to be happy, contented and living as the person you see yourself as. Nobody can tell you what you need to do, some people will tell you what you secretly want to do.A situation I have never been in myself having transitioned, very young.
As you get older, you might eventually come to subconciously resent somebody you love, for holding you back even though it is your decision.
Gender identity problems are a recognised condition. Getting your wife/partner educated regarding the condition is essential, with understanding she might be accepting of the inevitable and support you, even if it is only as best friends. It's a fact women marry men, who they see as supportive, protective, otherwise they would be lesbians. Every time this particular thing happens, there is a lot of soul searching and heartache.
I have done a lot of research into the causes of this condition, a lot I have posted as sticky topics in the general forum.
A typical example of a an acrimonious case, very upsetting. http://gendersociety.com/forums/topic/10112/divorcing-and-the-unreasonable-transexual
I suggest before you do anything, you both go to see a gender specialist.