This is excellent. However, what I first loved about this piece on reading was its ambiguous nature: what was it exactly that you wanted to express in this letter? Love? A guilty admittance of something? Or maybe just that a feeling had fizzled out, sadly. But when I looked again and saw it placed in the friendship section, I thought that would probably suggest the content was of a positive nature, which I think takes something away (a little). If it was placed in the Miscellaneous section, there could be no guessing what this mysterious letter holds.
Wonderful write, though.
* Reading again, however, possibly you mean 'contain' like you cannot contain your feelings for this person, ie, an exuberant sort of love that no envelope could hold - and then, it should be in the friendship section, lol
sorry, I'm rambling
** double edit - I have literally just learnt a few days ago what the Fibonacci sequence entails!