How can I get my friendship back to how it was with her?

That's a tough situation to be in, I would say first and foremost you need to change how you treat your friends because having a "thing" with them and then hurting them just isn't cool, having a thing with them twice and hurting them really jerky. you should be lucky that this girl forgave you, she must obviously see something in you. glad you finally woke up though and realized your mistake. she was pissed at you cause not only did you hurt her once but twice! she took the risk cause she still liked you. anyways if you guys want to be close friends again you need to work out the problems between you two and I can tell you right now Don't Don't Don't get into a relationship with her, if you wanna work this out strictly be friends. be nice and understanding of each other, don't bring up your past relationship with each other as this will only make things awkward and worse. Good Luck