My mom is ruining my relationship.

Hi, I have no idea what to do any more. I have been in a relationship for 3.5 years with my amazing fiance. I am only 17, which I know is young to be engaged, but we are not in a rush to be married. Anyway, my mother has been lording her power over me - restricting time I can see him to only once a week, being extremely disrespectful towards him, and doing her best to control every aspect of my life. I have already graduated high school, have a job, and have my driver's license, though I have nI car, making it all easier for her to control. Every time I have tried to see him outside of her "boundaries" ( once a week in public and she has to take me) she yells at me until I cry and break down, and find myself feeling helpless. It is seriously causing tension between me and my man, he is sick of feeling like I care more about her feelings than our relationship. What can I do? It barely feels like a relationship any more and he is questioning my loyalty to him. Sorry for the long post, I am just torn up over this.