You don't say how long it's been since you and your partner broke up, but breakups involve the death or demise of a love relationship. As a result, we grieve lost love. All of a sudden there are changes to your social support system, your identity (e.g., being "x's boyfriend"), daily routine, perhaps even the place you live and your future plans.
Therefore, allow yourself the time to process what went wrong between you, especially if it was a bad breakup. Try to talk to a trustworthy, understanding friend who values your privacy so they can help you sort out your feelings. Just being listened to helps. Concentrate on your own happiness and becoming healthier, both mentally and physically. Make sure to exercise and socialize, even if you don't feel like it.
If you are in current significant distress or if you are still having difficulty after six months or so, consider seeking a qualified mental health counselor so you can identify and resolve any unhealthy behaviors or issues in your past relationships. You cannot control how your ex feels about you (her "hating" you), so just concentrate on yourself and moving on. Let her be in charge of herself and you just do you.