Within the fantasy genre, different races with vastly different lifespans are par for the course. Tolkien's elves are immortal. In the Faerun setting of Dungeons & Dragons, elves easily live 600 to 700 years, but are also reincarnated from one life to the next (except the Drow, sorry.) In the brutal setting of Dark Sun, they only live to 150 or so. Fae are immortal and ageless. In a genre where deities can fall in love with humans, the difference in lifespans is often magically equilized or swept under the rug and left to the imagination of fanfiction writers.
While that doesn't need to be explicitly on the page, it does need to be kept in mind as a romance progresses. At some point, the elf, fae, god, or goddess must consider that falling for this mortal, or shorter-lived mortal, will mean mourning them. It's a bitter shadow to the sweetness of a romance. No one in the throes of love and passion wants to picture life without the other, but how the characters approach this tells us about them. Are they the sort to throw caution to the wind, love now, and accept the consequences of grief later? Are they careful, guarding their heart until it betrays their good sense, and the specter of a future alone haunts every moment of affection? Do they push it away every time the thought occurs, willing themselves to ignore it?
There are as many ways to circumvent this as there are fantasy novels. Tolkien's elves must choose mortality with their love or immortality alone. In ACOTAR, Feyre was raised to the same status as Tamlin and Rhysand, making the questions moot. RA Salvatore and Wizards of the Coast had Catti-Bree reborn and reunited with Drizzt, although he spends four books navigating the world without her.
The other choice is to let the characters face grief, either explicitly on-page or implicitly by not offering a workaround. I have a soft spot for sad books and tragic endings. I'd read the unabridged Hunchback of Notre Dame several times before Disney ever contemplated giving it an HEA. The Silmarillion is one of my comfort reads. Characters navigating loss and grief offer me a natural catharsis to the struggles in life. To love and care is to risk the pain of loss.
It isn't something I've explored in a novel yet. My upcoming fantasy romance book, Mistress & Mage, pairs two people of similar ages and lifespans. My current work-in-progress does the same. Within my short stories, I've had a chance to, briefly, touch on it by bringing in an elf for multiple love stories. For Captain Caerue, the shorter lifespans of human, jaglin, and other races in the world, is all the more reason to take the risk. If someone will only live another thirty, forty, or fifty years, then it's time to love them fiercely and completely now, or the chance will pass. The brilliance of their time together is worth the darkness that will inevitably follow.
The story of him meeting and wooing Camellia (pictured above) is in the free anthology, A Season for Romance: Fall Flames, and is a sort of prequel to Mistress & Mage. Caerue and Camellia are the great-grandparents of the FMC, Delphine. However, I had to write a final epilogue for Cae and Camellia, because the darkness of grief can cast the light of love into sharp relief.

