I (M16) want to ask out my "almost something" (F15) without being cheesy but without being an idiot without emotions, how do I do it?

Look, I am a teen, I am in a relationship with a girl that is not official, we say "I love you" We hug, all that romantic stuff, but we haven't say "you want to be my partner" And well, I have to say it because oI promised to her that I would ask her, and too, we have been in this phase for already 6 months...

Now, the big problem I have is, I am too cheesy, I don't want it that way, she sees me as a little boy and I don't like it, I am a sensitive guy and I am trying to not be it, so I need a way to ask her without her thinking that I am a emotional guy (which I am BUT THAT WILL CHANGE!).

I was thinking in giving her a candy she like with a drawing of a character she likes, but when I say it too my friend, she immediately say that it was not ok, that I should write a letter or something more lovely and meaningful, and I get it, womens like that, but if I go that way, I am afraid she won't love me when I get my change once I turn 18.

What can I do so she doesn't think I am too delicate and so she doesn't think that I am too sore and unlovely to her?