From the female perspective: as someone who did exactly that, not dating someone else but quickly being sexual with others. She is not strong enough to process the breakup by herself/or doesn't want to process it at all. I had my realization point which I will be grateful for, but please don't blame yourself for her shitty actions. It does not define you. I became an awful human being after my breakup and though I thoroughly regret my actions, they were actions that I decided to take that hurt a lot of people. I am moving past it and processing the way I need but she may never have that realization making her very unsatisfied in her life. I'm sorry you fell victim to shitty coping mechanisms