I’m aware that elves love only one time and I am stickler for canon. Tolkien showed by making Miriel stay in the Halls of Mandos forever that it IS in fact possible for elves to have a second love if they’re so lucky and given the opportunity.
So clearly if an elleth or ellon decides to forever stay in the Halls of Mandos it leaves an elf open to having another partner at any time. If Eru wills that to be the case.
Then there’s also the case of Gwindor and Finduilas. While not married the two were engaged and on the verge of marriage and spending forever together when Finduilas fell out of love with Gwindor and instead fell for Turin.
Courtship and engagement is a very serious and long process for elves. (Especially in Valinor where there’s the luxury of time) and for Finduilas to decide that she suddenly no longer loves Gwindor is a considerably bigger deal than it’s made out to be..
So it’s possible either 1. Elves CAN in fact fall out of love with each other OR 2. They weren’t each other’s one as they so thought they were.
Oropher, Angrod, Aegnor, Finrod, Turgon, Fingolfin, Thranduil, Elrond, Maglor, Curufin, possibly Caranthir, Fëanor, Orodreth, were all married or have said to have found their one.
For most of these elves I write for were in love and I get that it contradicts the canon. But it’s fan fiction. Some of it is going to contradict canon to an extent, but I’m big on not stretching so far that it’s not believable.
I’ll leave the reader to decide which character fell out of love with who or which spouse or partner passed away. Or how they left the picture for Reader to come into the picture.