That's your answer. Speaking from experience, staying with her because you don't want to ruin your friendship hurts the both of you and is honestly selfish of you. It sounds to me like you both have some things to work on, with her it's her mental state and with you it's figuring out what you are looking for in a relationship. You said yourself you argue constantly and almost break up every time, so maybe you should break up seeing as how that is your inclination. I understand that you don't want to lose a friend, but you being unhappy in your relationship until MAYBE you don't feel this way anymore is not the right thing to do and will ultimately lead to you losing the friendship you were so adamantly trying to preserve.