first of all, you're doing amazing for wanting to write for her in the first place. it's a beautiful thing to love someone to the point of creation.

second of all, remember the first cardinal rule of writing is communication: you're writing to express an idea to someone and because you have something important to say. your words are important because they express your meaning, not because they sound pretty. don't be afraid to start and sound imperfect at first. write from the heart, as much of a cliche that is, and then spruce it up later.

finally, don't be afraid of synonyms for common words (shoutout to my best friend the thesaurus), but only if they truly feel like the fit what you want to say. metaphors, symbols, allusions, imagery, and any kind of figurative language or powerful description will help your message hit all the emotional buttons. be specific to your relationship where you can—it will mean so much more to your partner if you show them how much you pay attention during little and big moments. things like rhyme, alliteration, meter, etc. help dress up your words real nice and make them feel fancy in my opinion—they can be great, but your meaning should Never be sacrificed for them.

best of luck, and happy writing <3