Is a girl sinful for leaving the young man she was in a relationship with in suspense, after she repented and wanted to distance herself from him?

You did well to sever the forbidden relationship. You are not sinful for this; rather, sin lies in persisting in it, for such relationships are a gateway to evil and corruption. Be sincere in your repentance, do not pay attention to the whispers of Satan, and occupy yourself with what benefits you and brings you closer to your Lord.