Writer’s block? Read the love letters of ages past. the everyman’s library: love letters is a great source.
A useful framework is to articulate meaning in your path together.
If your letter sounds like a store-bought card, you might as well skip this entire exercise. Be specific to your reader.
A nice poem is a nice poem, but wordsmithing your own thoughts matters more.
Don’t make over-arching romantic statements if they do not reflect the authentic you. This should be true to your heart and who you are!