What will you do when you find your goodfriend betrays you?

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Recently,I found myself so stupid and deceived by my roommates whom I always believe in.She stole another roommate's bank card and withdrew all the money in it.After I told her that we had told school's secury department and the thief will be another girl we had always suspected,she told us it was her who stole it.We were all shocked and she said she didn't know what she was doing at that monent and asked for our forgiveness. I can't describe how sad I was at that monent and I found I knew little about her and I couldn't imagine how shadowy her heart might be. Now the relationship and atmosphere in our dormitry are so strange and unnatural.We won't come back to the happy days in the past. She promised that she would change the dormitry next semester. What can I do?