Wouldn’t you have wanted someone to tell you he’d had a girlfriend—who wasn’t you—all along? She won’t appreciate it and she might not believe you, but I think you should tell her. Even if she doesn’t believe you, it will open her eyes a little. She’ll see for herself soon enough. Don’t do it out of anger though, or as some sort of revenge. If you can, get in touch with her, tell her you need to talk to her over coffee or something. Be gentle and explain the situation. She probably won’t thank you. Or be very nice.