Corruption of the Heart by Jessica Manson
Lilith Carson is a normal girl whose parents died suddenly. She moves to a different state with her aunt she never knew. When she arrives she cuts herself off from everyone. She longs for friendship but doesn't trust anyone enough to let them in. Until one day when her crush appears beside her in the library. From there, they fell in love fast and hard.
Lilith finds out that when she turns eighteen she will transform into a Vampire-Witch hybrid. When a vampire conceives a female, that female is already chosen a mate for her. When Lilith was conceived there was a mistake. Her world is turned upside down when she must choose between two guys she barely knows.
Lilith has chosen the guy of her dreams and has settled into a life that she thinks is perfect. But nothing is ever as it seems. She will soon face the ultimate betrayal by someone she loves deeply.