I understand your feelings and rationalization of the situation. I can only encourage you to be truthful with yourself and it sounds like you are regarding your feelings but not your actions maybe. Please focus on your own happiness and less on how others feel
or on what is supposedly "normal". Your ex bf is not forced into anything. He chose to be where he is. So did the girl, mentally ill or not. Get some distance so that you can evaluate clearly. Best of luck!