No need for any formality; might as well just start it off bluntly.

Why me?
You could have so much more, but you settle for me. I mean, I barely reach your shoulders. :)
Bad joke, yeah. But really, I never figured out why. Or how. Was it like a "she's the only thing left" sorta deal?

Maybe I'm just being insecure. In fact, I probably am. But none of this makes sense; I've never gotten a reason. But then again...I'm not sure why I need one. See why I'm confused? I ought to shut up and be happy and complacent about having you. But can you really blame me for being so amazed? You're so...unbelievable that it took me a good two minutes trying to just think of a word to describe you. 

See what I mean? Loving you makes me so dumb. It's like you've stolen the part of me that lets me think normally. My thoughts were always pretty incoherent, but you've made them absolutely insane. It's like you've opened the door of a bird cage - the birds immediately fly away and are impossible to catch. You'd probably laugh at my childlike naivety, and sometimes I really do feel like a child around you. I used to pride myself on being some independent, self-sufficient bitchy (sorry) person that could stand up for herself and even someone else if they needed me to. But now, you've got me relying completely on you. My moods revolve around what you say and what you do (I almost started tearing up when you sent me that thingy you wrote to Sam, it was that sweet).

You've changed me into the type of person I've always wanted to be. You're such a good influence. Yet I'm so scared that one day I'll accidentally overwhelm you and repel you. In fact, I'm scared of that as I write this. I want to stay with you for as long as humanly possible, because you're just that astounding. I don't know what I'd do without you.

I'm in love with you, Edward.