Fill in what you know about her and leave the rest blank. The more specifics you give it, the trip, the argument you both still laugh about, the more it sounds like you wrote it. Treat the result as a first draft to copy out by hand.
Read it aloud once. Cross out the lines that don't sound like you and rewrite them. The point isn't to send this letter — it's to break the blank-page paralysis so you can actually write the one that's been sitting in your head.
See some examples → A handful of real love letters I've written my wife over the last decade.