It is very common for folk to fail to translate a physical descision to an emotional one, and it sounds like neither you or her has carried out that step.
If she is harrassing you, then she is building an image of you out of emotions, and not personal experience. Yes, this a form of slander, but in the realm of personal responcibility, she is accountable for the actions and words she carries out/speaks and writes, and you, likewise, are responsible for yours.
Between the ages of 11-21 there is a lot of social development going on- especially in the way you define your boundaries. by forming a relationship those boundaries have been intermingled, and they are not honestly designed for tearing apart again, in an ideal world.
Since the event of parting the relationship has already happened, you'll need tactics for handling the past.. as the past. Speaking about it in everyday terms as done with will definately help.
There has been a family relationship I have had to treat in this manner myself, because of an emotionally abusive effect from it.
One of the tools that has helped me is the understanding that each person here on the earth is temporarily on loan from the origin of our personhood, God. Between a mother and a son, for example, the son is loaned to the mother as a family member, for a relationship. Sometimes that relationship blossom very well, and sometimes it doesn't. There can be personality conflicts, poor choices, jealousies and abuses- but that does not change the bigger context- both of these people still come from one source, one being, one bigger and better relationship.
Outside of that context a person carries a lot more responcibilities than they truely ought, and can claim territory without rights to it-
As, for example, your ex still claiming emotional rights to you.
In this situation, your feelings of goodwill and adoration to her are a null point, because she means you harm.
That is not a position of heart that either God or Jesus will stand for, as our maker desires that everyone love -and forgive- eachother with the same bigheartedness that our Maker shows us.
If a person (like for example your ex) recognizes that these actions are wrong and seeks forgiveness, then there's a chance to remedy the situation, on a level.
If she does not, then it is best to handle the situation 'tough but fair'- close all connections, do not follow her on social media, and if you are in social circles in everyday life where she is active, stick to your story and do not react in the public emotionally to it. She is using emotions to bombard with, and responding in kind acts as kindling. Do not try to appeal to her better nature or at all respond, and there is a likelyhood of this wellspring of vehemence drying up.
In the meantime and the personal level, it's vital to have the bigger picture in mind- she is answerable to her maker for these actions, and whether willing or not to percieve it, is acting out of line. Is God not big enough to handle both you as an individual, and her as an individual?
If you had intercourse (sex, and yes, this covers everything from kissing to petting to stuff done in the nude), then the emotions are entangled. I reccommend speaking with a person older than you and trustworthy, about their experiences with breakups and how it was handled. Knowing that people around you, physical and real, have survived this sort of situation and adapted to life after it, will help show there is a horizon and life available afterwards.
It is possible for you to use techniques to help yourself keep calm in the thick of the storm.
I reccommend first of all, that you take time speaking one on one with a person 5+ years your elder, so that the burden is lightened (even if this is a school counselor). Speaking about it with your mouth face to face can help keep it from becoming a damaging and lingering burden on the mind, because when you let an emotional problem linger, it tend to wear grooves in your personality in the years to come.