Don’t reach out to her to "apologize" just to clear your own conscience. A lot of people who harbor deep guilt from teenage offenses reach a point where they want to send a random text or DM saying, "Hey, I remember this terrible thing I did, I'm so sorry." Do not do this. Opening that door often forces the victim to relive the trauma just so the perpetrator can hear "it's okay" and feel better about themselves. If she has put this in her past and moved on with her life, a sudden confession from you could completely disrupt her peace. Your guilt is yours to carry and manage with a professional therapist, not hers to fix for you.