Good-Bye, Valentine (Heartbreak Killer Book 1) by River Gardner

Mel was happy, engaged to the love of her life, planning to spend the rest of her life with him.

In an instant, her entire world was completely shattered, her heart ripped to shreds. She's left to pick up the pieces, and put herself back together, but how?

Then, one night, one traumatic event, unleashes a monster from inside. A monster she can't seem to control. One she's ready to use to seek revenge on those closest to her, who destroyed her. Now, she's ready to destroy them.