I wouldn't worry about structure or anything too formal, just write how you feel. I like getting letters from my girlfriend, it's very sweet, there's nothing there for him to not like!
As it's Valentine's day, or any other 'benchmark' day, perhaps recount the nice things you've done recently/in general and what youre looking forward to for the rest of this year, then comes the soppy how much he means to you bit.
Presentation is more important than structure I'd say. Some nice girly or colourful paper, pretty pens and the like all add to the effect!
Good luck!