It is a hard cycle to break. I dont have any great advice, I am sorry! But welcome to pc. Is there any way your mom would be willing to buy different things, when she does buy the food? Like if you let her know that type of food is really difficult for you or something? I am glad your fiancee is trying to help you, as well, maybe things will get better when you marry? (have a date set yet? )
I know this probably didn't help but I feel for you.