Dark Flame by Alyson Noel
Once it seemed being immortal was a gift to Ever Bloom - now it's a curse. And she's a danger to herself.
Ever has been dabbling in Magyck - seeking to explore her own powers, hoping to find a future for her relationship with Damen. But she's not in control - in fact, the magyck has bound her eternally to her immortal enemy, Roman. Whenever he's near, she feels his heartbeat, his breath, his blood racing in her veins. Ever is in deep - and the one person who might be able to help her must never find out what she's done . . .