Dear Android, you have been given excellent advice, here. I say, PLEASE, please, please, take all of it to heart.
If you don't, I fear that both you and your "friend" will be heading straight for Heartbreak Hill. As owathu pointed out, you have been cheated on, yourself. If she will cheat with you---she will cheat on you if the conditions are right. And, vice-versa for you, too!
Both of you need a couple of years (at least) of focused work on self awareness and growth. You can have more fun than you have ever had while working on yourself. Just do not commit yourself to another until you have become a whole, mature person, It isn't fair!!
dandylion