She was the first person to truly befriend me and show me kindness during a time when, in my head, I had no family, friends, etc. Her complete selflessness and kindness saved my life as I had planned to attempt again.

OP, it's time for some intensive counseling. Due to your past with your mental health issues, you've put your friend on a pedestal without realising it. You relate lots of positive feelings to her, which clearly impact the way you feel about her in general.

I don't know why I can't. But the major one is that I could never stand in the way of her happiness and cause a rift in her relationship with her family who she loves very much. I'm probably also too scared to lose her as a friend especially with our history.

Look, if your friend doesn't want to be with you due to religious incompatibility, this is her rejecting you.

If you want to move on, you need to go no contact with her and focus on yourself.