Hi Alex
Firstly, how cute are you?

Secondly, well there is a Discworld Companion you could buy, but it would take you ages to find anything remotely romantic I think. (Just to check - you weren't wondering if there's another language spoken on the Disc, were you? There isn't. It just seems that way).
You could mention the Disc's greatest lovers, as in the following quote:
Terry Pratchett wrote:The Disc's greatest lovers were undoubtably Mellius and Gretelina, whose pure, passionate and soul-searing affair would have scorched the pages of History if they had not, because of some unexplained quirk of fate, been born two hundred years apart on different continents.
Or the romantic film Pelias and Melisande:
Terry Pratchett wrote:Pelias and Melisande: A Storie of Forbiden Love. Thys wille shok you! With a 1,000 elephants!
You could liken yourself to Casanunda, the Disc's second greatest lover (has his own stepladder).
Or you could just say that you love her more than:
Sam loves Sybil
Detritus loves Ruby
Carrot loves Angua
Nobby loves checking door handles.
I think it's safe to say that any love poem with a Discworld theme probably won't be romantic. But good luck!
What's up with this glass? Excuse me? Excuse me? This is my glass? I don't think so. My glass was full! And it was a bigger glass!