first, please please please go see a doctor as soon as possible.
talk to your dad, let your sister talk to him, talk to your mother if needed, do whatever it takes to see a doctor, as you need urgent medical treatment. you are having a depression and you need medical support, the deeper your sink, the harder it will take to lift you up.

second, try to understand your mother's point of view, love is blind, and she may be right. If you feel that you can get over whatever problem your mum is warning you about with your fiance, then make it clear to everybody. Stand for your life. fight. I'm not sure how this works in your culture but I'm sure there could be a way out.