The best love letters sound like only you could have written them. Add the memories, details, and little things that make your relationship yours, then use the result as a first draft.
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.