I am feeling sadness that you became a temporary "band aid" for this girl who had not known what she wanted (and probably won't know herself and her emotions with any certainty for awhile). You were enchanted, I suppose and tricked by this girl into becoming involved. You liked what she let you believe about her: maybe some of what she revealed to you was true and some of it was entirely false. It feels bad to trust and learn that we were deceived. This does not make YOU a bad or foolish person. It means that this girl is confused about her own identity and seems not too concerned whether she hurts the feelings of others.
Is it important or necessary that you tell your family? Do you have a therapist or counselor you can speak with who will keep your information confidential?
So, you are saying you feel a preference for other girls and are not ready to speak openly to anyone about this. There is nothing wrong with you. Trying to force yourself to like the opposite gender (guys) because this is expected of you will just make it difficult for you to form ANY kind of relationships with guys -- and there's nothing wrong with having a friend or two of the opposite sex in your life strictly as friends -- as long as this is made clear from the start.
You know who you are and what you want. When you feel confident, you will speak openly about your preference to others -- including family members. There may be at least one person who will not accept and will criticize. Be prepared for this and know that they are the one with problem and not you. Again, the world is filled with people who act unkindly for reasons that even they do not understand. You might find this girl comes back begging for another chance after spending some time with guys -- but I would be very reluctant to allow this person back into my heart and trust, because she's likely to keep bouncing around in a state of indecision for awhile, which will keep you feeling insecure and upset -- you do not want this. You are not the answer to helping her make up her mind. You need to be strong and keep your energy together for yourself and understand that there are many girls who are able to love and who, like you, have made up their minds. Give it a little time and you'll find you've learned an important lesson from "miss undecided." Remember...this happened to Ellen DeGeneres, too!