Hey there!
I think "Brown" and "XDad" have it right when they say it may take you some time to comprise a letter. In fact, I would advise to do so as your feelings must be coming at you so quick right now and it's hard to get a grasp on all of them (at least I know it would be for me). Perhaps stepping back a bit and thinking on what you want to say and how to say it is a really good thing in the midst of all the powerful emotions that come with this stuff.
I got scammed by someone through reunion.com. It was a nightmare (though that wasn't the sites fault).
Also, if her mom isn't returning your letters then the direct approach might be best anyway.
Keep us posted okay?
Much peace your way,