Aside from vocabulary, an analysis of the quote evokes a number of literary conventions, which find their source in traditional Japanese culture.
1.This seems to be a standard manga or dorama style declaration of the speaker's feelings for another person お慕い申しております (the sentiment) and 気持ちをお伝えしたくて (the confirmation of the speaker's desire to convey said sentiment)
2.Request -- if the other person would agree もしよかったら -- for "the person's hand in marriage", since official documents are traditionally exchanged, as in a contract or, at least, here, a formal vow, an "eternal promise", i.e. 永遠の契…