You should start by making a list of things that you want her to know. This will help you to convey your message in a comprehensive manner and you will not forget the intentions of this letter.
You better formulate a theme of your letter to keep your thoughts organised. Decide what you want to tell her first and where to mention that particular thing which irritates you. Tell her how life has changed after meeting her.
Don’t be formal as it looks fake but it does not mean that you should not open the letter with proper salutation. It is always better to mention the date at the top of the letter so that she can remember this special day. Address her by saying “My Love”, “Sweetheart” or any other title which shows your deep love for her. You may call her with a name that you have given her.
Start the body of the letter by explaining the reason of this letter. Give her clues about your intentions but do not disclose everything in the first sentence. You should make her realise how important she is in your life.
In the next paragraph you will tell her what is it that has made you her admirer. It can be anything like her personality, nature or even just smile. Whatever the reason is do not feel hesitation in telling her. Then you will tell how much you love her. Say it in your own style for example you can use a dialogue from a romantic film that you watched together, piece of poetry or may be a passionate sentence from your heart.
Close your letter in a romantic manner by signing off with sentences like, “you will not age for me” or “you will always have my heart” etc.