Hi there. Not trying to stalk your threads, honest. They just keep popping up in the unread posts thing :p
So if I read correctly, you just came to either realization or acceptance or both very recently? 2 year old account aside...you weren't ready then for whatever reason I guess?
If I may, I'd like to relate to you what happened when I realized that I'm female inside. First I sort of internally posed the question "Am I trans?" to myself as an actual possibility. I spent the next 3 days in my head without eating sleeping or really doing much of anything. I cried for joy at the possibilities, I cried in anguish over the lost time and girlhood. I mourned what was basically the death of my male self.
While that sounds a bit overly dramatic, the second I "realized" that was pretty much it for me. How do you go back once you realize everything you are is a facade?
I was on an emotional roller coaster, a floodgate of emotions was opened. Things that had been suppressed my entire life were let out into the light. All of my emotional defenses were predicated on masculinity, so they got temporarily disabled as well. In short, I was a raw nerve for quite a while. From time to time I questioned if I was bi-polar because the mood swings were so severe and all over the map.
Things have settled down now. I'm developing new defenses and coping methods. And as I posted in your other thread I'm finding fulfillment on a level I never dreamed was possible. That tempers the darkness greatly.
SO! That brings us back to you. If I can throw your thoughts back at you here :
I can't do that - Can you afford not to? Think about what can be gained, not what might be lost. If you're miserable then what good are you to others, and how long do you think you can hold onto what you've got? Nothing is permanent. Things WILL change, independent of all this. It's up to you to influence that change to make your life what you want. "What if I fall?" "Oh my darling, what if you fly?"
You shouldn't post anything to others, you can't even handle yourself - If people could only speak to others in universal truths then not much would ever get said. Throw in a disclaimer if you have to but as long as your heart's in the right place then that's all anyone can ask of you.
My wife will never believe me - The power of denial can be quite strong. She may not. Or, if you and she share an intimate emotional and intellectual relationship she's probably seen signs already that may come to make sense as she works through this mentally. Perhaps she discounted them, perhaps not. The only way to know is to talk to her. Keep in mind that in the end her belief doesn't change the truth.
You will ruin her life, she planned on growing old with you and she has plans - This is not your fault. It's not like cheating. This was not a choice. The only choice involved in the matter is if you share your secrets with her or not, now that you're fully aware of them. It doesn't make it any easier, I know, but perhaps it can frame the internal debate a bit. This is one of your bigger and more complex issues and one you may have to work very hard to not let drag you down. Or she could be hugely accepting, stranger things have happened

She put in 25 years and three kids with you - You put in 25 years and three kids with her, while suppressing who you are on the inside to fit into a mold of others' design.
You don't even know for sure, do you? - In your head there are lots of reasons you aren't trans. But in your heart?
Just go back to being what you know - This is a "fun" one. It lessens with time but you've spent your life answering this call. It's safe. It's a known quantity. It's what you think the world wants from you. And it's completely miserable on the inside.
You can't, she knows you are seeing a therapist and you promised you would talk very soon. What are you going to say? - My advice (without knowing your wife, obviously) is to be honest and try to put things in a way she can understand. In my experience cis people just don't get trans-ness. You may have to educate her on gender issues. Their gender matches up and has therefore never been an issue. They don't understand what a mismatch does, or that there can be a mismatch. Or maybe she's super empathetic and progressive and will be supportive.
Start out putting things as simply as possible, be clear. Even if the only thing you say to her is "I'm a female on the inside. I always have been though I didn't, I COULDN'T, realize it at the time." that will get the ball rolling. How she responds is the unknown. She might get scared, angry, who knows. Personally I would avoid "backing down" to ease her anxiety, though that may be seen as cruel. But to do otherwise is to deny yourself or delay the inevitable when you have to come back and be like "Remember when I said I was trans and then said no not really? Well...really."
What will you do now? - Move forward. Don't rush it, but don't dwell in the dark places either. Figure out what you want in life (like that's so easy, right?) and then make it happen. Your views and feelings will continue to evolve rapidly for a while, so don't stick your neck out too far but whenever you find the quicksand you're standing on has solidified a bit go ahead and take that next step.
And please don't feel like I'm pushing you into transitioning or doing anything you're uncomfortable doing. Maybe your next step is to take up fishing as a peaceful hobby, or performing altruistic acts to help you learn to love yourself. You have the innate human right to pursue happiness. But you've got to catch it yourself.
So...this post is incredibly long and I feel like it's a huge personal failing that I can't ever be brief, and honestly I feel like I'd be better off to just delete my response. Perhaps point by point advice like this is way off base and not what you were looking for at all. But I'm going to post this in a sincere attempt to help and to offer hope, because there IS hope. For all of us, no matter how dark things may seem in the moment.
If anybody reading this needs an ally, please don't hesitate to PM me.