Josie, it's rough - I know. I'm in the same boat as you: kinda knew that something was wrong way back when (and before the Internet, damn it!), and had no idea what to do other than suppress it and go on with the normal meet-a-girl-and-get-married-and-have-kids thing. What else was there to do back then? We had no idea.

And here I am, middle-aged, teenage kids, going through the craziness you're going through.
Here's my advice. One step at a time. Although I'd love to be right there with you and wake up and throw on a wig and a dress and stomp around the house proclaiming "This is who I am, so deal with it!", it's not realistic to do that. We made our choices - however wrong they were - and it's not fair on the family to do what we want to do. But it's not fair on us to be forced to live with decisions we made decades ago either – a family isn't supposed to be a life sentence without parole.
My strategy is this, and it may or may not be helpful to you. I'm going to stick with being male-me until the kids are older (i.e. college). Then all bets are off. At that point, if I've raised them well, they'll be old enough to understand and even appreciate the fact that I kept all of this mess inside for their sake. The spouse? Er, that's the problem. I'm not too fond of her right now, so I'm not going to be sorry to see her leave me. Your mileage may vary.
It's a long wait. Years. But that doesn't mean I can't start preparing. I'm losing weight, trying to stop drinking, trying to exercise, practicing my voice in the car each day, working on my body language, researching, reading, reaching out, and just enjoying every little moment I have alone with myself. It's not perfect, but it's a start - and it's better than nothing.
In five years, I'll be mentally ready to transition quickly (if that's even what I want then) and catch up with everyone else. And to be honest, unless you and I transitioned in our late teens or early twenties, we'd be no further ahead than we will be in a few years. We'll still have many years - decades - to live truthfully, to make friends, to form great relationships, be pretty, be who we were meant to be. It's not a race, although it might seem like the most urgent thing in the world right now. It's so tempting to just throw in the towel and come out of the closet fully, it really is. But after so much careful thought, doing so would be so destructive. That's where the selfishness would come in – not the fact that we transition or come out, but the fact that we are so desperate to do so that we trash everything and everyone in the process. There is a middle ground, I'm convinced of it. And I'll damn well find it!
For someone single and with no kids, this is all hard enough. For those with families, it's a logistical nightmare. And nobody sets out on this journey with the intention to hurt anybody else. But hearts will be broken in the process, and that's unavoidable. Even those with the simplest lives break many a heart in their transitions.
The way I see it, it's selfish of me to want to live life as someone who others think I'm not. It's selfish to want to transition. But it's equally selfish of everyone else to want to keep me caged up and living in this horrible body, unable to express who I am. They'll get over it if I transition carefully. If I stay as who I am, I'll go to my grave regretting every breath I ever took.
Again, there is a middle ground where you and your family are balanced. It will not be perfect. It'll alienate many friends and family members, but it'll make new ones in the process. We can't please everyone, but we can do our best. And one person who deserves pleasing is yourself. Don't leave that important factor out of the equation.
PM me if you need to talk about this offline.