After getting married via the Ceremony of Eternal Bonding, you might find yourself contemplating a divorce. These things happen. It is possible to annul the marriage in-game, either before the ceremony or afterward. If you have already scheduled your ceremony, then you’ll need to be in possession of your promise wristlet. Anyone who has already had their ceremony will need their Eternity Ring.
Whichever stage of the process you’re in, take your wristlet or ring to the Sanctum of the Twelve in East Shroud. Speak with the white-robed Sanctum Acolyte next to the Ceremony outfitter, right next to Claribel for “The Ties that Bind” quest. Trade your promise wristlet or your Eternity Ring to the acolyte. This will effectively annul the marriage for you and you only. Your Eternal Bond partner will need to do the same on their own. Even if they don’t, you are still free to marry someone else.
You may keep your mount from the wedding ceremony. But unfortunately, if your previous partner does not dispose of their Eternity Ring, they will still be able to teleport to the aetheryte nearest to your location in the open world.