Which comma rule does this student need to apply to this passage?

I really thought she loved me but I discovered her grave betrayal one afternoon. It was easy to figure out because she accidentally sent a text to me that was meant for him. I was confused by the text because it said, "Don't tell Mike." I felt insulted that she thought she could get away with this infidelity. At first I felt very angry but then I felt so hopeless and depressed.