Blood Orange by Karina Halle
Once there was a man who fell deeply in love with a woman he could never have. When their affair was uncovered she was brutally murdered in front of him, and he discovered he was cursed, doomed to live forever as a vampire.
Over the centuries, he found his love again, and lost her again, until he was so broken he gave up on love entirely, sinking into the depths of depravity, losing his humanity.